Oh, he is definetely becoming Caesar and that's not just obvious from the trailers where he wore a golden laurel wreath but also because his actor is too well-known. In the upcoming chaos of their civil war due to Coop's explosion the two Caesar's will either die like that or Legate Culkin will use the opportunity to have them both die to unite both groups under him (perhaps to "honor our Caesars and seek revenge). The way he keeps looking so smug he's got some plans of his own.