I wouldn't be surprised if someone like Sou snuck in to become Tomu's nibante
Eritan becoming Rantomu's nibante is highly unlikely to happen...they are the 81th batchmates.

BTW the things have started to look a lot like the mid-2002 situation haven't they ?- when Osa became the youngest top otokoyaku (with Asako, one year junior, as nibente) and there were a bunch of talented upperclass stars clustered in shin-senka.