Ubi should keep working on the existing factions; they've got an interesting world they've built, they should expand on that lore and work on involving it in the gameplay.
I'd hate to see For Honor become a pimp for every possible warrior fantasy. "I want to see a warrior with a sharp stick!" "But I want to see a warrior with a different, sharper stick." The less historical accuracy we see from here on out, the better. The centurion, for example, already deviates from history by not supplying the warrior with his shield. These are fantastical renditions of the historical article. I think Ubisoft should spend their resources on bettering the three factions they already chose to present their combat engine.
The secondary nature of the game's lore has already turned away countless players--I know a lot of people who don't care about the game because there's nothing to care about. As much as I would argue there's countless things to like, it's not an argument I can win. For Honor, at the moment, is a glorified demo with very pretty maps.