Random encounters from wandering monsters are the classic hazard, but more mundane problems are the mainstay of travelogues.
What I'd like to do is have a probability system for testing for different categories of hazard, adjusted to account for the players' decisions about how to approach the journey. For example, if they're hustling they should be more likely to rush into unsafe terrain and slower to notice creeping hazards, like strange gases or sunstroke. If they take time to examine and maintain their carts and tack, they should be less likely to suffer malfunctions.
Basically I think I'd like to have a "how do you spend your time and energy?" decision. You can travel at absolutely full speed, which makes all kinds of problems more likely to arise: walking into danger, exhaustion, injury, spoilage, accidents and so on. You can take excellent care of your mounts and gear, but travel slowly and use up both food (in travel time) and resources (for maintenance). So you're deciding what your priorities are.
I think (based on my mate's requirements) what I'll suggest is that each day has six 4-hour blocks, and you can choose how many to spend travelling. Those you don't, you can use for downtime activities. Travelling for more than 8 hours beyond this counts as Forced March, no matter in which order you do things!
Travel speeds, and their bonuses and penalties, are otherwise as in the DMG.
Okay, I've pretty much run out of subsystems now, so I need an actual main system... curses.
Right, let's have a stab at this.
The basic idea is much like the random encounter roll, except without monsters. There will be several rolls made each day (or whatever period), with various modifiers, to determine whether anything unusual happens.
I'm probably going to change that because I think it's too faffy on reflection, but let's take a look anywway.
There are five categories of random events: Travel, Health, Gear, Animal and (of course) Encounter.
Each roll is a 2d10. All rolls are modified by party Morale.
Travel rolls gain a +5 bonus if anyone successfully Planned.
Gear rolls gain a +5 bonus if anyone successfully did Maintenance.
Animal rolls gain a +5 bonus if anyone successfully did Animal Care.
Characters roll Health individually, modified by their Health.
I'm not going to work up actual tables here because I suspect this model is just too faffy. Essentially, it would be low rolls resulting in serious problems, and high rolls resulting in either nothing or some small benefit (such as faster progress).
In this somewhat simpler model, there's just one roll to determine whether the notable event of the day is good, bad or indifferent. A secondary roll determines the type of event (such as Travel, Health etc.) and its effects are then calculated.
Roll 2d10 (modified by Morale) for the day's events to determine whether things go well or badly. Then use the highest of the two dice to determine the type of event that occurs. The DM is responsible for deciding exactly what has happened and (where appropriate) framing it as a challenge for the PCs.
If the group is forced marching, they suffer an additional -5 penalty on the roll.
Events allow rolls to avoid their effects. Activities performed in downtime and current status modify these rolls.
- 4 or less: Calamity! Something has gone seriously wrong. The party will make little or no progress today, and may suffer lasting effects.
- 5-7: Problems. Something significant goes wrong, and requires considerable effort to deal with.
- 8-10: Minor setback. The party will lose a little time, energy or patience.
- 11-15: Steady progress. There are no particular problems today.
- 16-17: Good going. The party makes better progress than expected.
- 18+: A stroke of luck!
Use the highest of the two dice faces (the Type Die) to determine the type of event from the chart below. For example, if you roll 4+7=11, the Type Die is a 7. If an event isn't appropriate (for example, the party has no animals with them) use the other face. If that also isn't appropriate, it's a Travel event.
Most events allow an Avoidance Roll to ignore the effects - only one character may make this roll, and in most cases the DM determines which. If the Avoidance Roll is successful, the DM narrates the issue that arises, but the party manage to overcome it without any significant difficulty: they pick a safe path through the marsh, fend off illness, notice the damaged reins in time to avoid an accident, and so on. If not, the party must deal with the issue in-game.
One further point: for some events to be meaningful, we need to assume that waving hands and chanting isn't a solution to everything. This is because with very little else happening, clerics can cheerfully cast cure spells all day. D&D's hit points are a handwavy mixture of stamina, resolve and physical injury. The simplest thing is to assume that, while clerics can easily heal battle wounds or virulent diseases, they can't do much about low-level illness or the time wasted when someone gets hurt. Maybe the gods just don't consider it serious enough? Injuries and illnesses still take time and exhaust the afflicted.
- Animal event. Something happens involving one of the party's mounts, pets or pack animals. The event can be avoided with a successful Animal Handling roll (DC 20 - Type Die), with advantage if the party did Animal Care last night.
- Health event. The party member with the lowest current Health is unwell or injured. The event can be avoided with a successful Constitution save (DC 20 - Type Die), with advantage if the character received Healthcare last night. If the unHealthiest party member is already afflicted by a Health event, choose the next unHealthiest.
- Hostile encounter. The party encounters active antagonists, ranging from petty thieves to bandits to vicious monsters depending on the scope of the event roll. This is handled like any other encounter and there is no avoidance roll.
- Gear event. There is a problem with some aspect of the party's gear (or, on an excellent roll, their equipment helps them progress faster than expected). This can be avoided with a successful Perception roll (DC 20 - Type Die) with advantage if the party did Maintenance last night.
- Travel event. The broadest category! An issue arises with the weather, roads, navigation, terrain, natural hazards, other travellers, local residents, the authorities, or perhaps the party simply find something interesting to investigate along the way. The DM should choose an appropriate avoidance roll, typically Survival or a social skill (DC 20 - Type Die).
- Health event
- Animal event
- Gear event.
- Travel event.
- Travel event.
A Hostile result isn't necessarily a combat. The party might choose to lay low while a raiding horde passes by, or plan a way to pass through a spider-filled forest without alerting the creatures. The difficulty of the challenge should reflect the event roll result - a Problem shouldn't be bypassed with a roll and no real loss of time.
But how does all that work?
Okay, how's this supposed to work? Here are some suggestions.
- 4 - A mount is badly hurt - it trips and injures a leg, sinks into a bog, is poisoned by a roadside plant, develops infected sores from poorly-fitted tack, or is attacked by an animal in the night. The party might choose to abandon the creature and keep going (redistributing possession as necessary), or stop travelling for the day while they rescue, tend and reassure it.
- 5 - A loud noise, strange animal or other surprise sends the pack mules racing off into the forest. The party will need to hurry to round them up, once they get their own mounts under control... and there are plenty of places for a mule to disappear.
- 8 - A horse is unusually irritable and badly-behaved after weeks of travel. The party are slowed down as they struggle to keep it moving as they want. An Animal Handling roll won't deal with this problem, because that's what the AH avoidance roll represented - they've had their chance.
- 11 - The horses feed from a cluster of strange herbs, and spend the rest of the day twitching and whinnying, but it has no serious consequences.
- 16 - As the party rests, a grazing animal wanders aside and reveals a hunter's track, which proves to be a useful shortcut through a difficult area.
- 18 - a ranger's companion bounds aside from the path, leading them to a suspicious patch of fresh-dug earth. A few minutes' digging reveals a small iron-bound chest containing silver pieces and a magic scroll.
- 4 - The route through the hills proves a mistake when the weather worsens, leaving the party slipping on wet scree and struggling against violent gusts. A pack of supplies is lost when a party member nearly falls from a narrow ledge. (Mechanically that's probably going to be food, but it could include some party gear as well. The DM might allow an attempt to find and recover it, but that should be a long and difficult task)
- 5 - The party enter a farmstead to ask for news and buy supplies. Instead, they find a secluded religious order who are angered by the intrusion, and by something about the party. The zealots order them off their land, and all the farms across this valley belong to the same unwelcoming group. It may not be possible to travel through this valley at all.
- 8 - after a shortcut through a thicket, the party are constantly plagued by insects, due to the lingering scent of certain leaves. Their progress will be slowed and patience frayed unless they find a way to escape the flies.
- 8 - a large band of bandit-hunting soldiers orders the party to halt for interrogation (Persuade or Bluff avoidance roll to quickly convince them to move on). A lengthy search and questioning delays the party and potentially inflicts some minor damage.
- 11 - unstable stepping-stones plunge someone into a stream, but thankfully nothing is lost.
- 11 - the party are forced to detour when they find a landslip has wiped out the cliffside path they hoped to take, but manage to make up lost time.
- 16 - A break in the trees on a hilltop offers the party a splendid vista of the landscape ahead, helping them plan the rest of their journey. They gain the benefits of journey planning for the next 1d3 days without spending downtime.
- 18 - The party encounters a band of wandering traders who are glad of some company. The traders can provide skilled Maintenance and give them advice about the route ahead. The party have a friendly contact in the next settlement they visit.
- 4 - the party member is afflicted by food poisoning, and completely helpless. Even with magical healing, they will be too weak and exhausted to travel or pursue downtime activities. They can only engage in absolute necessities (i.e. they can still do combat if necessary). They cannot act in downtime until they make a successful Con save (DC 12-days elapsed).
- 5 - the party member wrenches a knee on unstable ground, and can only move slowly. The party's progress is reduced today.
- 8 - a fever affects the character's senses, so they see and hear illusory threats, and act erratically. The party must decide how to respond.
- 11 - a slight headache from the biting wind makes minimal difference to the journey.
- 16 - whether fresh or exhausted, the character feels unusually clear-headed and focused. Their attentiveness helps the party avoid difficult ground and push rapidly on with their journey.
- 18 - food the party gathered proves to be especially refreshing, and after the meal they all feel in good humour. The party's Morale increases by 1.
- 4 - A broken wheel leaves a cart useless. The party must choose whether to make long, difficult repairs (a challenge planned by the DM) or abandon the cart where it lies. If they have a spare wheel, they must hoist the cart (a dangerous job) and try to fit the new one.
- 5 - Vermin have found their way into a pack, chewing the bag and eating supplies. The party loses 1d4 daysworth of food.
- 8 - Cooking gear is damaged by poor packing. Unless the party has spares, someone must burn a downtime slot to fix the gear or rig up alternative cooking options.
- 11 - A poorly-fitted sadly leaves someone saddle-sore, but otherwise fine.
- 16 - The expensive telescope someone insisted on bringing allows you to spot a broken bridge up ahead, and the brigands apparently lurking nearby. You can take a different route and make good progress, or you could attempt to confront the bandits.
- 18 - you have exactly the obscure item you need to earn the gratitude of a passing wizard, who offers you a warm welcome at her well-hidden tower. The party can rest and feast well tonight, and has the chance to obtain some minor magical supplies.
That seems more or less workable to me?