I don't know if you guys know yet, but it is confirmed now that the faction limit is 30+ rebels.
May I suggest adding the Papal States, Portugal, Persia and perhaps Georgia.

Thanks and good luck with your mod, I was really looking forward to the RTW edition, but M2 should provide a better base.

PS, you may wish to contact Ogniem i Mieczem TW or Pike and Pusket TW to cooperate as those guys have lots of stuff on Poland-Lithuania and Muscovy that you may find useful.

Cheers.