So Roman Empire will have a different NI set than "Byzantium"... Interesting. I guess Roman Empire will be formable even without being imported.
Maybe. They did also mention the Jomsvikings will get their own NI's, and there's absolutely no reason to think they'll be formable in EUIV without importing them from CKII. Entirely possible the Roman Empire is much the same.
EDIT: Nevermind, the achievements quite clearly indicate reforming the Roman Empire will be doable in EUIV.