I think they could bring back Wayne Palmer. I don't see why they didn't have him as President in S.7, other than thinking it would be clever to have a Madam President, because when the season was written, it looked like Hillary Clinton was going to be the next POTUS in the real world. I like Taylor but I would like to have seen more of Palmer minor.
But David? Nope, great character as he was, he's dead as a dodo. The Tony thing was stretching it but bringing back Palmer, who we saw take a bullet in the neck, before seeing his corpse, would stretch it beyond breaking point.