And please, PLEASE: Do restricted take-downs. What I mean is the problem of both Thief and Hitman(less and less with Blood Money) and to an extent Chronicles of Riddick, the reason why stealth is almost always the best option when available: You are not penalized in anyway with successful and constant knockouts, and indeed you can use the same winning formula to take out enemies just as long as you keep some arbitrary "stealth meter" full, no matter that in reality making a whole complex full of people pass out or silently be forgotten to dark corners(dead or unconscious, it has so far hardly mattered) would be next to impossible, or at least highly improbable thanks to radios, guard change routines or just plain old human wits(="I guess it was rats""olol, a dark closet, tam-te-dam, just walking my oval here, la-la-laa!").
Don't take me wrong, I absolutely adore stealth games, but rather than giving stealth gameplay mechanics that force you to interact with opponents in often forceful manner in an obstacle course, have mechanics that instead try to hammer in the idea that the best way to stay unseen and unnoticed is not go anywhere near the enemy! I mean, throwing coins or rocks to distract drone-like guards, or flicking a lightswitch(!) and then passing unnoticed works well in an environment that presents a set of tools to circumvent openings in a system, just like in a puzzle, but I would prefer if stealth was unnoticeable and subtle, a welcome addition to a wealth of methods(like gun-ho and infiltration). A game that caused you to mix and mash different approaches to problems, instead of taking along a sort of a strict "stealth class" or a "gunner build" in an otherwise classless system.
So far, only Deus Ex has had anything at least remotely similar. And even the god-game suffered from having a god-stick similar to Thief's blackjack; the nightstick. You could very well shut down whole complexes of hive-like office hierarchy and high-tech augmented wetworks with just your nightstick and a set of multitools. Gee, I do love the feeling of power that I get from being undetectable and uncatchable, but it's slowly starting to lose the shine it used to have.