I don't think that two planes are automatically better than one.
What the 2-plane mechanic created were both choke points in an otherwise open map and at the same time backdoors to be aware of.
That made them an interesting gameplay feature - not the mere fact of having more than one map.
Rules were also slightly different. Myrran had magical roads but also a more dangerous - if richer - environment.
I don't see why WOM should not use the good elements from that while discarding the annoying and clear workarounds.
Since maps are all pre-generated anyway, nothing would prevent the game from having an entire "magical and dangerous" continent.
It would be divided from the rest of the game by the ocean with only the odd flying / waterwalking creature straying from it.
That would also make ships useful in any way.
Alternatively it would be surrounded by a magical border that you have to breach first. That would require magic and ships.
To conquer that continent would mean a greater possibility of magical resources / shards, temples... whatever. Kinda like defeating the Guardian of Orion.