Do whatever you like -- the riskier the better. This is a rare opportunity to let your creativity run free; I'd hate to see it wasted on demands of Planescape/Alpha Protocol/New Vegas/Mask of the Betrayer 2, or anything else that you've already touched upon. The breadth of premises we've seen from you so far that have achieved cult status should be an indicator that everything you touch automatically turns to gold -- but, honestly, you should know this by now. Why else would you make a Kickstarter based on the Obsidian/Chris Avellone brand?
This also means you can:
- Establish a whole new IP without worrying about whether or not it will sell, since a project like this is guaranteed to make its money back before release
- Spread your budget evenly from beginning to end, instead of frontloading your game with the most amazing two setpieces and loading the rest with filler/backtracking
- Put the focus on character interaction and storytelling, and not the same "tough white guy has weapon + regenerating health" power-fantasy garbage we're assaulted with day-by-day in the games industry
For my money, I'd love to see what you guys would do with something outside your wheelhouse. A Banjo-Kazooie-style platformer with Obsidian writing would be the rare game I revisit every year -- but those are just my tastes, of course.
The major themes I'd want, regardless of genre: Interesting characters, dense settings, bright/saturated colors, and good voiceacting. Literally everything else is up to you -- and you probably don't even have to do that much, since I'll probably still buy whatever you put out.