REV2 is more that 2 years old and only added 2 characters instead of 6, so I'm pretty sure we're getting X4 next. They probably started working on it a few months before REV2's release, but I think it'll be announced at EVO Japan. I feel it's still too early for an announcement at EVO with Granblue Fantasy Versus releasing soon.
I don't think they will cut any character. It seems the next part of the story will be set right after Revelator, with both That Man and Ariels being jailed. If they always reboot the roster when they reach 25 characters, the number of missing fan favorites will just keep growing. I also don't see them making new models for everyone, it will probably be like GGX to GGX2.
What I'd like to see:
Universal hitbox when in hitstun for the same weight class, so we don't have to learn a different combo just for Zato or Potemkin
the three heavy weight classes merge, so we only have light, medium and heavy
No Roman Cancel in neutral
Removal of danger time or it is balanced (not triggered by clash+RNG)
A few new characters nearly as easy as Sol, so newcomers are more likely to stay and don't all choose Sol or Ky. It would also be easier/faster for old players to use multiple characters.
Bridget, just so we can listen to "A Simple Life" again. Testament too because his gameplay is pretty interesting. Under Night cl-r was rated for Switch and PS4 in Taiwan in May, so it's very likely to be announced at EVO. I guess Granblue Fantasy Versus will get a release date and a few more character reveals.
https://gematsu.com/2019/05/under-night-in-birth-exelatecl-r-rated-for-ps4-switch-in-taiwan