I agree with you; I'd like to see several kingdoms outside of Hyrule and technically, Hytopia is a different kingdom which - surprisingly - isn't located in a different parallel universe. In order to keep the lore consistent, it could work like that: Naming the world/universe Hyrule while there...
I guess they want to keep the lore of Hyrule being the world consistent. Because some sources do say that Hyrule, as the sense of the world "Hyrule", being the universe in which Link and Co are living in. It's been further supported since Termina is basically a parallel world.
I hope so as well. Maybe we can even explore the lands beside Hyrule which we couldn't access before. Either way, they're probably developing on BotW2 ever since the last DLC of BotW, so that's a lot of time spending on this sequel.