There are enough realms for 9 total books.

uhhh I'm just gonna let book 3 play out first

If book 3 goes well Jotunheimr, if not I think Asgard

we already visited Asgard though, it was full of bunny costume versions of Alfonse and Sharena for some reason though...

Since Midgard is pretty much the Summoner's home world, does that mean that we could be visiting our home in a chapter?