>>10580606There are lots of other smaller castle-like structures and iconic features throughout each game's world that would make for good smaller sets. Wave 1 sets could have Hyrule Castle as a flagship set, wave 2 could have Ganon's Castle, and beyond that they could make up original castles, or dig into other Zelda games for larger sets, Clock Town, Skyloft, Bridge of Eldin, Twilight Realm, Windfall Island or any of the four major settlements from the Breath of the Wild games would all make excellent large scale sets.
Honestly, I love classic castle but outside of random one-off sets it's never going to be a big theme again, not for a while at least, so Zelda might be the best we could hope for. Zelda really has the potential to be the next long-running castle theme.