Brainstorming several different approaches. Goals: should provide interesting trade-offs while respecting the fictional logic.
- Chance of spell failure. Maybe N in 6 chance, where N is based on the armor heaviness (light = 1, medium = 2, heavy = 3). Problem: the rational course of action is to carry a suit of armor and put it on after all spells have been used. This is lame. Such an approach might even encourage annoying things like taking off armor to cast spells and then putting it back on (yes, this can be balanced with random encounter checks, but still lame).
- Wearing armor causes an armor penalty, equal to the type of the armor (where light = 1, medium = 2, and heavy = 3). This penalty applies to all physical checks (attack rolls, saves, ability checks, etc) and works much like the encumbrance penalty (and in fact is cumulative with it). This could be offset by a class-based armor skill (fighter = 3, thief = 1, magic-user = 0). Problem: while this would increase the game cost of magic-users wearing armor slightly (-3 to physical saving throws, escape rolls, and constitution checks associated with drowning are big deals), it doesn’t really impact casting spells at all. Further, one seeming corollary of this system is that zero level characters would probably have armor skill of 0, complicating the common case if consistency is maintained.
- Use a roll-to-cast system. I love this, but it’s also very invasive, and probably requires reworking many spells as well to do correctly. I would like a solution that supports the traditional Vancian system.
- Maybe magic-users just don’t get as much benefit from armor, but still take all of the downsides? Kind of like this. Magic-users could get AC 8 from leather, AC 7 from chain, and AC 6 from plate. Has promise, but is perhaps too complicated. Also, I don’t much like the corollary that magic-users have less skill with armor than standard zero level civilians. I prefer to think of all the character classes as somewhat competent adventurers; magic-users are not assumed to all be frail academics. I suppose zero level characters could also gain less benefit from armor, but that seems to introduce unattractive complications.
- Magic-users can wear armor, but only special enchanted armor (elven chain perhaps). This necessitates some justification for why magic-users can’t cast in normal armor (such as overly scientistic claptrap like how metal interferes with magic, which just invariably leads to subversion of the balance rule through creation of things like wooden or chitin armor). Also, it means that all “fully upgraded” magic-users must aspire to finding a suit of special armor (kind of like how cloaks and rings of protection are so critical in AD&D). Suboptimal.
- Learn to stop worrying and love magic-users in armor. I don’t mind this aesthetically. In fact, I quite like sorcerers in armor (picture by Stefan Poag), but it does seem wanting in terms of class balance (greatly decreasing the relative combat power of the fighter, specifically). That’s not the end of the world, especially as I have already improved the to-hit rolls of fighters in my current game.
- Provide compelling alternatives that compete with armor (such as robes). Combine with options 6, and maybe a increase the encumbrance cost of armor while not adding a full more physical penalties on top of the general encumbrance penalty. This is close to status quo, with the exception of explicitly legitimating magic-user armor use.
- Class-based AC. This has a certain attraction, especially if taken to the logical extreme where you literally say that fighters and clerics just have AC 3 no matter what they wear, thieves have AC 7, and magic-users have AC 9 (aside: clerics really should be the AC 5 class, but whatever). That solves the problem in one sweep, maintains both abstraction and balance, and makes armor only important as a kind of magic item or cosmetic affectation. That last bit is also maybe a problem though, as it does away with consequences of armor for things like drowning.
As you can see, my thoughts are all over the place. Anyone else have any good ideas or suggestions?
Edit: added option 8 based on Guy F.’s comment on Google Plus.