I was thinking post-apocalyptic style, with a few large, protected cities dotting the world. All the ratiation has affected the outlying lands and developed mutants. Then there is the biggest mutant of them all: a plant that is growing everywhere under the surface of the Earth. You get to take sides. You can help the cities rid the world of the mutant creatures, restoring normality to people's lives and promoting the growth of civilisation again. Or you could help the mutants and allow this new life to grow and either mingle with or dominate the normal humans.
Technology plays a big with Cybernetic Implants, personal Power Plants, the ability to construct automatic mechs and pilotable tanks, weapon modification, explosives of varying sizes. Psionic abilities can be learned. And the Covert-Op (thief) can obtain as much experience as the others by using other methods.
I've come up with the following classes. Names are in progress and more accurately reflect their ending skills.
Matrix - extreme Offense and Defense
Big Guns - extreme Damage
Covert-Op - knowledge and bombs
Elementalist - psionic weapons and healing
Human Tank - extreme Resistance and area effect skills
Demon - The Hulk meets Magneto
Any thoughts?