I was thinking it might be neat to work in some holograms or something like that for the TOS actors (a la "Unification III"), and they can play their actual ages now, or just make them up to look older. Regarding others, how long do Travelers live? With Will Wheaton hosting all the after shows I wonder if he's interested in reprising his role, though they might be saving that for PIC.
I definitely could see VOY's Doctor, or perhaps a Dax. Braxton from VOY. Perhaps they could bring in Phlox or an El Aurian (though I don't think they live a thousand years). Future Guy, Sphere Builders, Daniels, are all out there, as is Harry Mudd and TNG's Rasmussen. How long do Borg live? Though Seven's on PIC, they might be able to build a crossover episode around her. If not her, the Borg Queen (after she's done with PIC). I think Mirror Universe versions of characters are open to play around with, and it would be a nice tie-in to the Mirror Universe comics to finally see some TNG Mirror Universe characters in action.
I definitely could see VOY's Doctor, or perhaps a Dax. Braxton from VOY. Perhaps they could bring in Phlox or an El Aurian (though I don't think they live a thousand years). Future Guy, Sphere Builders, Daniels, are all out there, as is Harry Mudd and TNG's Rasmussen. How long do Borg live? Though Seven's on PIC, they might be able to build a crossover episode around her. If not her, the Borg Queen (after she's done with PIC). I think Mirror Universe versions of characters are open to play around with, and it would be a nice tie-in to the Mirror Universe comics to finally see some TNG Mirror Universe characters in action.