I think by "Late Era" we could just mean further down the trail of buildings. The final line of stables might offer you teppo horsemen. (And yeah, that'd add more cav, which is always a good thing! :D )

In 1600 around the time of Sekigahara Japan had gathered a lot of weapons. Including guns, a lot of guns, and cannon. So later in the game, (Not a era per se, but certainly later.)

Ugh, what's wrong with you CA. I want multiple eras, and the ability to play as any clan. XD

Seriously, the ability to play as only a few clans seems like a serious issue. What about Japanese players who want to play as the clan that controlled their province? Or were actually their ancestors? To have them in the game but un-playable just seems ridiculous to me. Hopefully it'll be modable.