You're asking the real questions here.I disagree, I think the other key characters need further depth to their arcs without being constrained by the plot devices caused by Xehanort.
What's Roxas and Xion's purpose now that they are complete beings? What was Terra and Aqua's past like when they were younger prior to Xehanort's involvement? There's a lot to explore now that we're in a new saga.
I actually had the idea of Ephemer and Skuld being playable characters from the X credits. Alternate worldlines are a thing now so the excuse to make them playable is right there.
I also wouldn't mind seeing Ephemer and Skuld going on a adventure. Anything that involves more screentime for Skuld has my stamp of approval.