Dystopia:

It won't be d20 based. I play a lot of Pathfinder, but I'm actually not super keen on the d20 system (I don't hate it, but it's not something that would work for this game since it doesn't have that “old school” feel). I've borrowed some of the things I like about it, but it's going to have its own system. The primary rolls will be made with a d10 + modifiers, so in that respect it does share a large similarity with the d20 system. But there won't be things like ability score modifiers: if you have an 18 in something, that's what you add to your rolls instead of say, +4. I'm trying to keep needless math to a minimum so people can just focus 10% on calculations and 90% on playing their characters and having fun.

Magic will also have MP rather than spells per day, and I'm trying very hard to make sure caster Vs. martial class balance doesn't get out of hand in later levels like it can in d20. Casters are going to be very powerful at mid-to-high levels, but this will be balanced out with a very tight MP budget, so spells will have to be cast carefully. Some of the bigger, nastier spells will cost as much as half your total MP, so you'll have to really think about whether the big payoff is worth risking so much of a resource. You don't regain MP after an encounter unless you have items to do it with; otherwise, if you run out in a dungeon or something, you're out until that particular roleplaying scene is over. MP regen items will cost more than your standard HP potions. They also won't get a ton of HP, and armor and weapon options are slim. So while spellcasters will be powerful, versatile and extremely valuable in party, they will need to work together with other classes to really be effective. Being able to call down a meteor storm isn't really helpful when the other party's fighter gacks you in two turns.

Fevian:
Noted! I didn't really think about that. I have some replies to do here, so I'll probably take your advice once I'm done with them. Thanks for pointing that out!