Not sure how viable this is, so I'm just throwing it up here for fun.
The basic idea for the class is this:
The class starts out with high HP per level (+5) and one or two trees that provide basic combat viability (but not much else - maybe two or three talent trees total, all relatively common melee-type stuff). You also start out with a Generic tree ("Soul-Stealing"). The first talent on that tree will let you "mimic" a single talent of an enemy you encounter for a limited period of time (turn-based, potency of mimicked ability and duration increase with talent level).
The second talent is where the tree (and class) get interesting: for each level of that talent you take, you'll be able to use the "soul-steal" ability on an enemy once (the one-time use is irrevocable). This ability will allow you to take one talent tree from any that the victim knows, which means you will be able to create a "mix and match" class of your choosing. The downside to this ability is that each time you use it, you will suffer a penalty of -25% HP (current, not original - i.e., you can't go down to "-25%" HP by using all five talent points, you'll just have your pool reduced by 25% of current total each time). Each point taken will also reduce the amount of HP you receive per level by 1.
Players will need to find the best balance between getting as many good abilities as possible while still trying to preserve their overall HP pool and choosing synergistic talent trees.
Class Idea: Soul-Stealer
Moderator: Moderator