So here is the thing:
Obsidians games in the recent past have been...well okay
But Obsidian has by far the best wrighters in the industry!
So yeah, give me a game "like" Planescape (not necessarily Planescape 2)
Give me a game where I can talk my way out of most fights if my stats are high enough.
Give me a game where I actually have to choose if I become a warrior or a talker (unlike f.e. Fallout 3 where you can be the smartest person in the world but also duo-wield fatmans (exaggerated))
Give me an interessting setting, don't just make me a Farmerboy who rises to become the hero of the land, but give me an interessting character (like in oh I don't now PLANESCAPE!)
Also: If it is necessary: **** Voice Acting!
As much is i "love" it, it is the reason why we wont see a game like Planescape again, because recording is expensive and why record douzands of lines people most likely wont ever hear?
So last but not least: Make a game you really want to make. Don't make it because "*sigh* the people want it, so I guess we should make it".
I count on you Obsidian, rise to become a hero! Bioware has disappointed many people with Dragon Age 2 so this is you're chance to become the hero of the downtrodden! (Yeah, I exaggerate again)