A SD FRPG is something I would greatly enjoy. There is ONE primary thing SD could do to make a truly memorable entry into the hall of fame. Implementing this would lift the bar way above anything we have seen so far. Those of you who know my core posts here know exactly what i am going to suggest. Create a game with a tool set that allows for maxim user / player customization.
Design the game, from the ground up, to be easily modifiable by players. This means, a relatively easy to use editing tool that allows players to edit the game environment at a deep level. Fan made mods (quests/adventures/cities) add a tremendous replay value to the game. Further, it harnesses the desire of many players to create quests, environments, etc. that push the envelope. Such a set of tools will unleash the (dread) 'bear cavalry' in all of us.
Consider what they did with the elder scrolls franchise. The 'construction set' allowed for awesome replay value. I am not suggesting an 'oblivion' upgrade or clone. I am suggesting SD create a FRP game that includes a set of tools so we, the players, can use the same engine / environment access that the developers of the story used to create their FRP story.
As far as the detail of the actual 'campaign' created by SD, I would suggest three things:
1) maintaining the aura of mystery as the story unfolds.
2) many interesting choices with real tradeoffs.
3) player enabled decisions that actually affect the game world in a substantive manner. Neverwinter nights 2 (Mask) comes closest to what I envision.
I hope this is a helpful addition to the discussion.