The Covenant were very well known for hunting down Forerunner artifacts and expanding their technology with them. Coupling that with their bespoke, “coach-built” approach to ship building (made to order), it would certainly be the case for a Prophet's personal flagship, like the Solemn Penance seen in Halo 2. Regret's Assault Carrier was known to have taken shots from the orbital SMAC array on it's way to the surface of Earth and lived to tell the tale, which suggests she was outfitted with advanced Forerunner technology, as the only other known ship to take such a beating was Didact's flagship Mantle's Approach.
This city-esque variant of the Assault Carrier will be our new Covenant titan.
(Props to Lord Stark for suggesting this very thing a while before we were ready to accept it.)