there are "rumors" that are pretty solid.
The last two Factions are going to be "Wood Elves" and the "Mages Guild" (this is not their official names but they will be some variation of these two).
Dont take my word for it though...as I said, its not "official" yet!
Regarding the OP Suggestion, they are already doing that somewhat. They wont be introducing an "Egyptian Faction" but they are adding Champs that look like them. Tuhanarak for example has a clear "old Egyptian" touch to her design.
Chinese will be represented in Shadowkin (or already are). Shadowkin is an "asia" faction not just Samurai faction. Norwegian or Viking style Champs are a bunch in Barbarians and even in Orcs to some degree.
What they will probably NOT do is introduce "named" Champions, like Gods from Greece "Ares, Zeus" or any known Characters from historic records. They will make "lookalikes" but not the character itself.