Well - let's examine what little we can.
Vivienne has thrown a ball presumably to what exactly? Celebrate the Inquisition? Petition the Inquisitor to join?
She's going to invite any Orlesian she thinks could 1st) Serve her agenda 2nd) Serve the Inquisition's agenda.
I'm REALLY curious about Morrigan and Vivienne's relationship at court... as I would find Morrigan, at first, to be horrible at the game - but quick to master it (which would be an excellent evolution of the character to be able to at least fake social graces).
I think it would be really cool to have Empress Celene make a momentary visit - or to visit, but then sit in a out of reach box where she can observe the crowd with her flunkies (and if you really work the scenario - maybe a meeting with her).
I actually could see Gaspard being there - as I bet Orlesian balls are neutral ground.
Who else is there all depends on Vivienne's agenda... more mages? Less mages? The Divine?
I could see Bodhan and Sandal there to possibly excite and entertain with "Enchantment!" It might be a stretch - but perhaps she's interested in Sandal. Nobility bring all sorts of oddities before court to ogle.
Teegan would be there I wager.
Shale maybe? Talk about an oddity. It would obviously be a long shot. But someone like Shale would actually be a great distraction during The Game.
I really hope ALL of your "current" companions are at the ball.
Flemeth would be a crazy twist. It would be amazing if a regal, beautiful woman introduces herself and talks with you... then Morrigan shows up and says something like: "Come along mother... let's leave these poor people alone." Or some such - without even referencing to the Inquisitor just "who" Flemeth is (of course, then you meet her later or something and she's the daffy o'le bat-brain she always is)
Whoever goes will tell Vivienne's story if - and that's how I'll play the Game and intuit not only what Vivienne "wants" - but how to either support to undermine that while using her for my own purposes.