Quest? Your character will probably be a Chosem One (TM) that is for some reason immune to Mind Flayer's magic/psychic attacks and will be able to extend that immunity on the party.
It would be nice to play the exact oposite: being an Elder Brain - and you need to direct your Illithid, and then control/subjugate "lesser" races for either food, sport or access to stronger psionics, (limited) magic resources or actual resources (bronze, iron, steel, adamantium, gems, rare gems, etc)
Maybe be faced with choices and consequences on the brains you choose to devour. The minions that you choose to kill. The few that you let live.
Maybe be faced with ethical choices: to pursue higher psionic powers, or go for necromancy. Punish necromancy. Use the big ship to sail across the planes in the quest to improve the psionics of your Illithid.
Then face the gith in all their forms (no refs. to Dakkon or sudoku), and have to adapt your strategies against them. I'm thinking something of the sorts of the X-COM style combat with an ant farm to take care of in-between "missions" or "scenarios" at home.
Have some episodes of descriptive CYOA for the story-oriented people, peppered here and there.
If I were Swen I would detach myself from the RTwP completely for this title - go TB - and then for a potential 4th installment bring back the RTwP just to "fuck" with the "fans", and to always keep the cynics guessing.