I guess those are still two plotlines for those characters , though I don't think there's any rush to resolve those. Terra becoming a master can be put on the backburner for a few games.
I agree it wouldn't fit Kairi's character and personality established in the games, but it would suit how the KH writing team has written her in the past sadly.
Let's hope for the best regarding our most neglected main character. I'm still pining for her to get a game in which she's playable.
I do agree that Ansem and his Apprentices will most likely play smaller roles moving forward. They might be contacted for Subject X's info, but I don't see their role being bigger than Ienzo's was in DDD.
Terra might stay back on the land of Departure to guard it, which could reduce his role for a bit. Aqua might as well, but I don't think it would be wise for them to bench their most popular female character.
Plus I doubt they'd let Ven go at this a lone so Aqua might be there for him on his journey.
I would prefer for Naminé to still factor into the story, just maybe in a supporting role, kind of like Yen Sid does.
Terra and Ventus "becoming Masters" isn't even a real important plot point anyways considering the setup that now lays out.
While Ventus will certainly be the central character of the BBS crew going forward with all the X-era stuff and his connection to the "X" girl from Isa's and Lea's subplot if she's indeed Skuld, I doubt that Aqua and Terra will take a backseat.
Those three have just been reunited and I doubt that Terra's and Aqua's strong protective streak for Ventus is gone so they will tag along with him and give support.
Except of course if Terra and Aqua become mission control characters with LoD as a base while Ventus travels with Lea and Isa.
Roxas and Xion might also be involved but I can also easily see them staying in Twilight Town with HPO and possibly Naminé, appearing as wandering NPCs when the world is visited.
Same goes for Ansem the Wise, Ienzo and the other apprentices, they will probably have a role that amounts to secondary support characters that make small cameos there and then to remind that they still exist.
Regarding Kairi I am literally at a loss since it seems that whenever they decide to let her leave the islands somehow she gets into trouble and has to be saved in some manner, also sometimes of no fault of her own.
If one looks at the epilogue and secret ending back-to-back, it looks like there's a setup for a split narrative at least for the first act of the new saga, with Lea, Isa, Ventus and the others having to deal with the returned Foretellers and the mysteries of the X-era including Skuld, Ephemer and Strelitzia.
Lauriam, Elrena and the other two will probably also pop up down the line somewhere.
The other side will involve Riku who's apparently looking for Sora (and as it now stands so far without Kairi), Yozora, the MoM, possibly the TWEWY cast and whatever this whole mess is additionally about.