I actually agree that having three setting books in a row is a little weird, and probably not WotC's first choice for scheduling. They might have postponed the Theros book if they didn't have to synergize with MtG releases, for instance.
Ahh, okay, that makes sense. Well, first off, it still doesn't rule out
Odyssey of the Dragonlords, as that is meant to take place on a lost continent that can appear in any world. And you will probably be able to blend it with Theros when that book comes out.
Second, you might also be interested in
Scales of War. It was originally published in
Dungeon magazine for 4th edition, but there's a full 5E conversion for it on this blog:
My experiences converting a 4th edition D&D adventrue path into 5th edition
5escalesofwar.wordpress.com
It starts small but leads to Tiamat's return, so there could be a chance to involve some characters from when you played through
Tyranny of Dragons. It also gets into some planes-hopping toward the end.