Maybe it's a bit too soon, but to be honest it's always fun to discuss stuff like this.
There is a lot of rich folklore and history in Japan to inspire a myriad of fantatstic warriors and creatures and I think what we've seen so far is only the tiniest tip of a very large iceberg.
I reckon the Shiho clan may be a given seeing as they're mentioned in the Prefecture starter set packaging but what else could we be seeing later on?
I'd like to see Yokai explored a bit more. There's the Savage Wave and Kitsune (gotta get her once my Temple starter is put together) but it would be cool to see a Tengu faction of some sort. Everyone who knows a tiny bit of Japanese folklore (or have played DoA 2) will know about them, they're one of the better known Yokai there are.
I think some Kappa could be cool but I can't see them getting a faction of their own. Maybe as allies for the Wave, or Temple even.
I liked Minstrel's idea for a Yakuza style faction, there's a lot of potential in Japanese history for more human factions.
Like Ninja for example. I know we have Bikou but surely there must be a Ninja group working it's own agenda in the Jwar Isles? I did wonder about Sohei monks, but they'd probably be in either the Prefecture or Temple.
There is one thing that got me thinking and that is Bushido is described as an Oriental fantasy, not Japanese fantasy game. Maybe factions with a Korean, Chinese or even Siamese theme may be present at a later point?
What's everyone else's thoughts on potential later factions? I know I'm jumping ahead but I've been wondering for a while now and started thinking up possible ideas. I had to mention them somewhere!