Who do you think will be the 11 Legendary figures and will that include the 3 Archmages (my guess is yes)?
I think it will certainly include Drizzt, Minsc, Tasha, Van Richten, Mordenkien, Dalamar, and Elminister. Maybe Fizban. Maybe the Nameless One.
I don't think Volo will be high enough level for this adventure.
I'm not sure who would be chosen from Planescape of its not Morte or Nameless One, Spelljammer, or Eberron (Vi? Who else can even leave Eberron for the wider multiverse? Vol? Lord of Blades?).