My first instinct is to say that this already happens, because as you kill things, you gain XP, and as you gain XP, you gain greater power.

There's also the idea that you gain someone's magic items that they have on them when you kill them...
Leadership also has a vaguely similar effect: you basically gain a second character. If, when you killed someone 1 level lower, you gained them kind of like a Cohort that was part of your body...that would be fairly balanced.
Gestalt, too, does something like this: if your main class is whatever and you have this feat, your second class just keeps getting added to you as you kill things with that class.
I'd also give my accounts of FFZ doing this:
#1: The Blue Mage. The crux of this caster is that they get hit with spells and learn them. This can be easily changed to "if you spend 1 full round "absorbing" the power of a creature, you learn it's ability." This hinges on a specific list of "blue magic" that an assortment of monsters also have. So, basically, there's a category of powers that exist expressly for this "absorber" to take, and those that aren't on the list, he can't take. This may involve adding these powers to other creatures, assigning levels to certain powers, etc.
For instance: there's the power of "teleportation" (roughly similar to, perhaps, a 6th level spell). The absorber can gain this power when he kills someone with it (assuming he's the proper level), but he can't learn the teleportation spell or other abilities with a similar effect. Basically, give critters exclusive powers that they can take.
#2: The Mimic. The base effect is that it can repeat the action of another, even without having the inherent skill to do it. It can also learn other characters' class abilities, etc. For this absorber character, that can be tweaked so that if it kills another, it can gain their "class ability" (at the opportunity cost of not having a different class ability), giving it a custom list of abilities that it can change out on the fly. The big thing is that they'd have a certain number of "slots" that they could fill. The abilities would be ranked based on the level they were gained at -- you have one "level two" slot you can fill with the 2nd level ability of whatever you've killed.
The basic thing is that the DM should be paying attention to the critters he throws at you, so that you have a nice mix of critters you can learn from, because otherwise you have no real abilities.
There's some rough ideas, anyway.