A bit over the top--though I do know a Traianos. He is extremely Greek, so he can get away with it. If you're that fond of
Trajan, it would make a nice quirky middle name. Be prepared (both you and the kid himself) for plenty of explanation.
Your combos
Trajan Octavius,
Vespasian Caesar, and
Vespasian Octavius are
really over the top, unfortunately. One imperial name/title per child is probably enough. ;-)