I liked the idea of Thief 3's "open city" concept although back then, technology was much more limited. With today's game engines and technology, it would be really cool if they made a MASSIVE open city, or a few smaller cities (i.e. like Assassin's Creed). Take Thief 3's idea and extend it slightly more by adding several RPG-like elements:
- Different shops in the city that sell different items at various prices (i.e. lock-picks, arrows, swords, cloaks, bombs & flashes, etc.)
- Bars where you can go to and hear the latest rumors and accept "side quests", completing these side quests can unlock special "underground shops" carrying upgraded goods and various safe-havens for you to hide against town guards
- More emphasis on non-combat equipment (i.e. different types of cloaks providing more stealth, protection, speed, etc.)
- Optionally make a very toned-down level up system (where Garrett gains very minor speed/combat improvements as he gains experience) - wouldn't take this any further though to keep things simple
- Make various banquets and parties where Garrett can "attend unnoticed" and steal food/gold/etc. He'll overhear about these random events when passing by the upper-class folk in the streets, etc.
I sure can dream huh...