Magic for All
I'm sure I've mentioned this previously, though I can't find it... one of my ideas for Monitors is that everyone's a wizard (or at least, every PC). There isn't a wizard class, or even a wizard archetype. The conceit is of playing "cyborg secret agent warlock space lizards", and having wizard and non-wizard PCs would undermine that, and the magic-plus-tech vibe that I'm trying to establish.
Another concern is that it's very difficult to balance spellcasters and non-spellcasters. There's a mechanical balance issue, because it's easy to end up with a system where spellcasters are just flat-out more powerful, either through sheer power, breadth of abilities, or flexibility (or all three). This means that they outshine other characters, and can also throw off the challenge curve for games: it's hard to produce an adventure that'll be an interesting challenge for people with very disparate capabilities.
A related issue is that spellcasters can also end up being more interesting than other characters, because they can do most of what everyone else can do, plus all kinds of cool magical things. Pre-4E Dungeons and Dragons suffers from this at higher levels, because warriors can fight things with a sword and take some damage, while wizards can fight things with magic, plus take some damage with magic, plus turn invisible, walk through solid rock, summon elementals, transmute objects, create magical shelters, walk unharmed through fire, enslave monsters, fly, and if absolutely necessary, try and fight things with a sword. This makes 2nd Edition's separate XP tracks absolutely necessary (and breaks in 3E without it). 4E D&D suffers from this because, while the classes are mechanically balanced, non-spellcasters' powers tend to boil down to "I hit it with a sword", while wizards are cheerfully petrifying, burning, teleporting, transmuting, and turning into clouds of sentient electricity that electrocute everyone they pass through. Reading through the lists of powers make this painfully obvious, as the designers' struggles to think of interesting names and descriptions for non-magical powers become increasingly apparent. There's just only so many "blows", "strikes" and "cuts" you can take.
So I'm looking at a system where everyone's a wizard, but I'm not yet sure how I'd want to handle this. How much of a wizard should everyone be? How much, if any, variation in magical capability do I want to offer? There are at least a few approaches to distributing magic, which look at both what magic's for, and how you get it (credit to Dan for highlighting this). Handily, I can think of three main approaches to each (obviously mixtures are possible). Today I'll look at the "what" side of things.
What is magic for?
The first way to approach magic is that it's a method. In this approach, magic allows you to do fundamentally the same stuff that you could achieve using money, force of personality, time, raw muscle or technology. Outside of combat, this kind of magic might unlock doors, start fires, provide transport, conjure food, smooth social interaction, heal diseases and so on.
4E D&D spellcaster powers are pretty good examples of this, since once you strip away the fluff, the effects of the spells aren't fundamentally different from what non-casters can achieve - magic is more inclined to use ranged attacks, and to target multiple enemies, but that's only a tendency. A sorcerer spell that hits a single enemy will tend to do the same damage as a ranger attack that hits a single enemy, and both may impose the same status effect. The main difference is that magic tends to offer a wider range of damage types, while most physical attacks do physical damage.
A disadvantage of this approach for my purposes is that I'm not sure it'd make any distinction between magic and non-magic beyond the fluff. A spell that zaps people with a bolt of lighting is largely indistinguishable from a gun that zaps them with a laser bolt, and this seems likely to encourage people to think about the the same way, and use them the same way. In a low-tech setting, lightning bolts can be the powerful (and perhaps costly or unwieldy) heavy weaponry of the party compared to arrows and spears; in a high-tech setting, though, it kind of makes sense for yer'actual heavy weaponry to handle that, so you're splitting a niche.
A second approach is more of a pegs-and-holes situation. In this approach, the game basically has a number of subsystems that are pretty much isolated. Magic is for handling magical stuff: it's of very little use outside that niche, and conversely mundane abilities probably aren't that useful when confronting magic. This might be a unique distinction, or it might carry through the game, so magic (weirdness), technology (tech problems), muscle (physical challenges) and brain (social challenges) are largely separate. In this sort of scheme, you might turn to magic when dealing with demons, ancient runes, undead, curses, spells and so on. However, if you're faced with some pirates or an alien menace, you turn to technology or practical skills.
There are some advantages to this scheme, because it intrinsically creates separation between magical and non-magical elements, which is something I'm after. On the downside, it seems like magic could feel like a bit of an add-on to the setting if not careful? That is, if most things are handled with tech and personal skills, and only magical issues are handled with magic, then it may feel like a bolt-on subsystem. This harks back to the old circular thief issue. Of course, the extent of the problem will depend on how magical capability is modelled, and whether it's only magic that's distinctly different.
A second problem with this approach is that it may create unwanted peaks and troughs, if there is also significant variation between characters' abilities. Characters with magic-heavy skillsets will be useless in non-magical situations, and vice versa. This is frustrating in the short term, and also means GMs have to try and balance the magic and non-magic elements of games so everyone gets to do something - a particular problem in short scenarios. I really can't assume that anyone's going to be playing a lengthy campaign in Monitors! However, if everyone has basically the same amount of magical ability, those issues will tend to disappear.
From a more general game design standpoint, it could also be difficult to design scenarios and challenges if magic is so different that other approaches won't work, because you introduce assumptions about the party's specific capabilities. This isn't that different from similar assumptions about skills, backgrounds or equipment.
A system whereby magical talent is modelled mainly as a skill or similar seems likely to fall under this approach. When there's magic stuff to do, you use your Magic skill; when there isn't, it's no use. But of course that's a generalisation.
I'm sure there will be games that work like this, but I don't know them.
Magic is weird
Yeah, I can't think of a better subheading.
This third approach is basically that magic lets you do a different set of things, which can't readily be achieved by muscle or technology. Not to say they're necessarily entirely impossible, but at least very difficult. Here the advantage of magic is that it cheerfully breaks the laws of physics, probability and so on.
A lot of D&D magic actually does work like this, especially things that aren't just attack spells. Haste and Time Stop interfere with the flow of time for you. Control Weather or Lower Water are outside the bounds of any but the most advanced technology. The various summoning spells conjure up creatures from other planes. There are spells to create extradimensional spaces where you can hide; to transform you into other shapes; to instantly heal injuries and disease; to selectively harm or repel creatures based on their ethical stance; to predict the future; to teleport between plants of the same species. Those are not normal things! They are weird. They are really not things you can readily do any other way.
A difficulty of this qualitative difference is trying to calculate how powerful spells are, especially for balance purposes. Is this spell that repels Chomskians more or less useful than the ability to track animals? Is turning into a bear more or less useful than inflicting triple damage once per encounter? This spell that makes surfaces slippery is pretty feeble until you're defending a steep hill in a strong breeze, then it's utterly lethal.
At the moment I'm inclined to aim for the third approach. Magic provides a different set of tools, which can be used in a variety of situations, whether you're dealing with magical or non-magical problems.