I think there's been plenty of citadel-like moments. The bants with Lambo and Esky, the last parts of the game with Ciri, the romance plots with Yen or Triss and that farewell with Shani in HoS, etc etc.
I'm sure people love the idea of having all the characters in one place having a good time but that's just not how it works in The Witcher universe. Each character got a proper send off and that should be enough. Everybody loved the part with Lambert and Eskel but do you really wanna oversaturate the good times?
Mass Effect had a rotating roster of squadmates, they spoke to each other in the elevators, made comments and had squabbles on the ship.
As far as I know, Geralt doesn't have a squad and getting aliens to work together and be friends isn't part of his job.
"But Darkstar, don't you wanna see your favourite characters again?", I've read the books, I've played the games and I've even watched the show. The characters are great but the whole Witcher universe needs a rest.
If you have too much of a good thing then all of it becomes meaningless. I'd rather not get sick of my favourite universe.