The more I think about it, the more I consider O’Brien a likely candidate, because (A) I don’t think it’s unreasonable to assume they were able to get Colm Meaney to do a guest appearance, and (B) because out of all the other possible legacy characters his at least has somewhat of a connection to Starfleet Academy, since they ended Deep Space Nine saying he went to the Academy as an instructor. So it would kinda fit to have him appear as a holographic instructor program or something like that.
The other character with a built-in connection to the Academy (or rather the Federation) would be Archer. I think getting Scott Bakula to finally deliver that speech we didn’t get to see in “These Are The Voyages” would be a neat touch.
Getting John Billingsley to do Phlox as a holographic medical instructor program would be super cool as well. Have him clash with the Doctor over some ethical dilemma.
Seeing another Dax host could work too, but would that really be “the cameo of a legacy character”? I do think the assumption is that a cameo will include the original actor.