Honestly, with the way KH3 ended, I can't imagine starting out the next saga with a spin-off. I feel like we need to start out with the original cast of characters before jumping around. Again though, I don't know if there's really such a thing as a spin-off in KH, it's more like Marvel, where the numbered titles = Avengers and the spin-offs = everything else. You can't really understand them without watching them all.
The only justifiable reason I can see for doing another spin-off before KH4 is to reboot KHX and UX to bring it to the forefront. It's too important to be left behind on a phone, and as they've remade everything else in the series into HD, I don't see why they would expect casual players to have to study up on the phone game to understand the newer stuff. They should treat it like Re:CoM.