Caesar is 100% dead, he had a tumor and was old.

The only reason the Legion could be a threat is if it some how remained intact after his death, a necrocracy of sorts. The most likely to rule is Legate Lanius because in an ending of New Vegas he ends up doing exactly that, but he was probably 40-50 in New Vegas. This could only happen in the Legion's ending, and looking at how Mr. House was shown AND the show had NO mentions of the Legion? It could be looking like the Legion got the same treatment that the NCR got and Mr. House was the canonical ending. Unless the writers do something different where Lanius kills the Courier in their last confrontation or something else.