Is this game PvP or PvE? Because if it is the latter, then it doesn't particularly matter what the players choose because, as GM, you're responsible for making sure they run into threats that are appropriate for their abilities- enemies weak to the burninating so they can die horribly, and enemies resistant to the burninating so they can force the players to think outside the box.
This was originally the vein I was thinking in, yeah- it'd be easy to make sure there's shock wolves and shadow wolves and thornfang wolves in amongst all the regular wolves.
If you expect players to be fighting players (or if you just want to be able to fairly generate enemies using the same system) then yeah, you could rely on the fact that since various magics are easier to resist players will be more prone to play physical classes, which in turn lowers the likelihood they take magical defenses which in turn opens the niche for the odd magic user, but THAT'S BORING. YOU KNOW WHAT'S NOT BORING?
Maybe adding effects to magic? I mean with physical there poison and bleeding with magic it magic so it could have anything (with in reason).
ADD EFFECTS TO EVERYTHING!
Okay, so, you may not have noticed, but I'm typing this while really tired, so I might ramble here. Basically, give all attacks two effects, a primary and a secondary. The primary effect is the obvious "I hit you with a sword" damage, but the secondary effect can be anything, including another source of damage. Thus, even though there might be a relative paucity of fire mages, their absence would be compensated by all the warriors running around with flaming swords that deal Slashing and Fire damage. I'm envisioning most weapons being focused around damage, while magic-users would have access to abilities that, say, deal Fire damage as the primary effect and the secondary is a DoT effect, or a multi-target special ability, or something. Which is not to say that they wouldn't also have abilities that deal Fire and Frost, or Fire and Piercing (Fire arrow attack!) or Fire and MOAR FIRE!
Then this came up, on both accounts. I don't have any
specific plans for PVP, but there's nothing stopping it and potentially players could be rivals or even enemies in certain tasks or situations, plus monsters are going to be built essentially like players.
The first part of your idea seems to be the opposite of what I want, though- essentially, you're saying to make fire damage more common, which would seem to make fire resistance more valuable, which would seem to make being a fire mage a bad idea because everyone's going to be resistant to it because everyone uses it and it's cheap.
As for the second part... still need to get around to abilities. The combat system is kind of too simple not to have them, and they're another way (alongside items) players are going to be better than monsters of the same level (since they'll have roughly the same stats otherwise), but I'm still not sure of the specifics.
Eh, it's more that once the bar has been lowered, it's hard to bring it up back up. People who put effort into their games wonder why they should even bother when anyone can run a minimalist RTD that people will be happy with. Why waste time and effort putting forth an intriguing concept, story, setting, or mechanical system you care about when someone will just make YOU ARE/DO/AT X?
Because those games are fun too? There's always an easy way to do things, and RTD's are no different. You'll note that most of the games that last a long time are the ones that do have good story/mechanics, not the minimalists. If you just want to have a lot of people chortle at your silliness, minimalists have a low investment cost to start and maintain. If you want to run your game because you like the story/mechanics, there will always be players who are drawn to the fact that you care. I'm kinda surprised I need to say this, considering we're on the Dwarf Fortress forums
In related news, many times I've been tempted to just start up some low-investment RTD with no/few mechanics. I can never bring myself to not add experience or something to it, though.