In more detailed way;
-Stealth could be composed of 2 factors: 1-being not seen and 2-being not heard. Both affected by skill, equipment and other factors, like light or distance to target, ground, speed of movement or even shoes.
- after losing stealth it could be possible to enter stealth mode again by moving out of sight and dark place, by using some flash/smoke bomb (like ninja) etc.
- opening locks, disable mechanisms and traps, trap detection.. setting traps, whole thief skills would play big part of the game and traps could be real pain in the *ss.
Generally, one screen could be like a chess, lots of tactics what to do, where to move, what to start with etc.
-alchemy stuff, not only as general help like in most of the games, but also major part of the game and tactics, which could include night-vision enhancement elixirs, poisons (not only deadly but also to affect target in certain way..like, heh, make him seek toilet, feel drunk, etc, without arousing suspicion), hearing enhancement (so you could get hear presence from some direction from greater distance or be aware that someone is coming), or thrown elixir which on impact relasess gas that reacts with oxygen and puts off torches' light in the close area.
-mechanical and other devices, like sound decoy that will make noise after some time, probing mirrors to look in to places before entering, other sound distraction things other than thrown item, eye gear, stealth clothes that absorb light, and other things that you could wear to look like commoner and don't get attention (if stay at distance).
-weapons, beside daggers and swords, also blowing tubes to apply poisons (chance of not getting attention), hand crossbows to apply poisons and damage, stun batons, other
Game itself could be placed in anywhere, from wide RP to single missions for money.