Interest check, heavily modified Pathfinder/D20
There are many concepts that I brainstormed in creating my system, many of which were just bad or horrible, but some I really liked but wouldn't fit, thus I figured I'd take Pathfinder and modify it with some of my favorite ideas. It also takes place in a rather unique setting of mine, certainly not a generic fantasy.
These changes include,
-Injuries and health. HP measures "Non-lethal" damage which is basically any damage that isn't a severe injury, thus cuts, bruises and similar. When attacked with lethal weapons/attacks, you make a defense roll to avoid lethal injury, success means only hp is lost, failure means you gain an injury of some sort, such as a broken arm, severed leg, etc. Much of it is mechanically overlapped to keep it simple to use.
-Advancement and power. I separated out power from level, so now you have Power level and Character level. Character level is about versatility, while Power level is about raw power (i.e. more damaging sneak attacks, more powerful magic, etc. Thus, you could reach level 100 and still be within a gritty power level though you'd be very versatile (though you could in theory start as unskilled but innately powerful superheroes). It would be xp based, but that is because you could learn a new class at a lower cost than learning a high level of a single class. This also means that certain stats, like bab, are no longer attached to class or no longer exist at all.
-Spheres of Power. Not my rules of course, they were designed by others, but this magic system is well developed and tested and fits my setting much better than slots. Be comfortable with this, as my setting has common low level magic. Even farmers use simple magics and warriors near always use it in some form. This is a setting where mundane warriors are rare and looked at as inferior. Luckily, anyone can learn magic. However, I do plan on casting magic to be skill-based rather than auto-success on casting (again, the mechanics are stacked for simpler play).
-Dice and Ability scores. The d20, ability modifier, and skill ranks (not other skill bonuses) are replaced by three dice. Ability score modifiers are altered to not only better suit this, but so that there are no modifiers less than zero.