I would say Shadow of the Demon Lord, but that IS level based. Still, it's worth a look. You pick Three different Paths (Novice, Expert, Master) of the course of 10 levels that give various class features. It's sort of a built in multiclassing that allows for a lot of variation. SotDL seems like a good system for "builds."
If you don't go Level-based though, the answer is usually Skill Based. I really like Warhammer Fantasy Roleplay, 2nd or 4th Edition. In that you pick an Occupation/Career which determines what skills you are trained in, as you gain XP, you upgrade those specific skills. Once you have upgraded all of the skills in a career, you are allowed to pick a new career and gain new skills. I think you can do this up to four times, at least in 4e. So your character might be a Human Soldier -> Road Warden -> Witch Hunter, or something like that. Warhammer as a setting is both very dark, gritty and dangerous and also comical and funny in a sort of british black humor.
There is also an alternative called Zweihander, which is built off WFRP 2e with it's own new features. It lacks the Warhammer setting, but is still very similar in built in themes, clearly drawing inspiration from Warhammer as well as other dark fantasies like The Witcher. Might be a better choice if you have your own setting.
Warhammer and Zweihander strike me as better for organically growing a character through RP rather than builds but I think you could do it anyway. All three games are very similar thematically in their settings. Darkness, corruption, blood and chaos. It's good stuff.