I was pondering a roguelike. Many roguelikes have you "build" your character over time. Even Nethack has skills, even if they're not vital in the case of weapon skills (but magic skills can make the difference between being able to cast a spell reliably and not). Incursion is based on the d20 SRD and has build options accordingly. I don't know how much ADOM's skills affect gameplay, but they're there too. Of course, most 7 Day Roguelikes don't have skills, as implementing a decent skill system takes time and detracts from the design goal of the game.
Anyway, I don't fancy skill systems. They lock you into one specific way to play the character (range vs melee vs magic vs sneak vs charm etc), and can serve as a trap for players who haven't played enough - if a Nethack player doesn't know what to spend skills on, he might waste skill slots on weapons as a wizard, or on bad weapons as a fighting class. And Nethack doesn't even deign to tell you that there is such a thing as skill slots.
So I was thinking of a system where "skills" are more fluid. Either allow generous retraining - you have X dynamic skill points that can be shuffled among skills - or base your abilities entirely on loot. An itemless character would just have the basic abilities of an adventurer - comparable to the protagonist of Rogue, but carrying a book of spells would instantly give him magical abilities - and then various implements could refine him further like in D&D4 (where staff, wand and orb wizards get different special abilities). The "wizard" could shed his magic items and grab a set of thief equipment or fighter armament instead, or even mix and match - a robe may boost spells, but real armor gives better protection in melee which is still interesting if you are carrying a Staff Of Becoming Ground Zero Of A Fireblast.
This would be relatively balanced with harsh carrying capacity rules (to prevent people from "multiclassing" too much by carrying extra items) and generous item drops (to encourage changing your setup without it taking half the game). Angband-style levels where levels vanish when you leave them would prevent extensive stashes.
Alas, Scrap already does this, but with the player being a robot scavenging robot parts. Nice game, try it.