I think Alfonse, Veronica and Sharena could make good cameo characters in some form or another. Hopping between realms is kinda their thing.
In general I think its important for the global reception of a character to factor in whether they return or not. Part of the reason I hold Asugi in such contempt is because Gaius was never as big a deal as Tharja or Cordelia, and winning one Japanese only popularity poll shouldn't be all it takes to win out over other characters. Tharja and Cordelia were a phenomenon when Awakening came out, but I hardly heard a soul talk about Gaius. In a more representative context he'd likely not have made the cut.
I also think its important for the character in question to make sense. Owain, Inigo and Severa were free to go wherever they wished after their ending but a character like Alm or Hector would be far too busy to go dimension hopping. Ike and to a lesser extend Lyn meanwhile do have room to emerge in all sort of places.
Purely as a meme pick I'd say Oliver would be very fun to see return. And him traveling dimensions in pursuit of art does seem pretty in character for him.