Day 4 mini update (missed Day 3, oops).
I threw together a quick chart to check result probability - the shift from d20 to d12 isn't actually as big as I'd feared, so I think I'm gonna go ahead with my initial idea of using all the dice (I also love the idea of throwing the d100 in there as an option, maybe there'll be an almost-guaranteed-fail roll). Also I've really set up my dice as a fail/success operator, which means the situations that call for dice rolls are going to have to be fail/success situations.
(Unless I set the target numbers as a range - eg. the PCs magic stat is a 2-6, if they roll higher they fail to cast the spell, if they roll in that range they cast the spell as expected, if they roll a 1 they 'crit' and get a secondary effect.)
I'm still working out what the situation of the game will be, mostly because I keep rolling different ideas around in my head. I'm leaning towards a fantasy game just to stick with what I and most gamers are familiar with (and any ideas I don't use I might end up implementing later - I wanted this to be variable game engine that can be used to write other games, anyways). I also definitely want there to be a win-state.