Urriak Uruk
Gaming is fun, and fun is for everyone
You know what...I could see any of these three, easily. Kamigawa could offer a very different genre experience, and expand cultural representation, a win-win. New Cappena seems a little too much like Ravnica and Eberron had a bad y to be really new, but I could see it.
Now...Dominaria. getting two whole card sets, one diving into the deep lore of the past if the Setting? On one hand, yes, it's another generic fantasy, but one they can use as a bit if a fresh slate, and it has millions of fans. That last part was enough fir Exandria to get a book. Particularly if the two Classic Settings are weird*Planescape and Dark Sun), I could see Dominaria making a lot of sens even with the existing free rules and older art book.
Yep, they all make sense to me as candidates.
I'll add, Dominaria makes a lot of sense if you wanted to include stronghold, mass combat rules. That place has a ton of wars.