Modernize the Infinity Engine, and create an accessible toolset with which players may create their own modules, assets, and systems. Sprite-based, fixed angle isometric graphics. Tactical party combat that can be paused to issue orders but isn't necessarily turn-based.
Effectively, Baldur's Gate with a NWN-like toolset. This could be a perfect platform for Obsidian to deliver their fantastic storytelling and premium modules without the expenses of 3D asset/engine development, and foster a creative community which would put out its own content with ease. With low system requirements, the engine could be made to run on a wide variety of platforms.
To keep the project small and self-sustaining, maybe adopt an episodic release schedule and payment system.
Note: This could be used to develop any kind of RPG setting Obsidian, or the community, desires.