What Universes Within would you want to see?

So, obviously, Wizards of the Coast isn't going to Universes Within every card they make, especially when they print entire sets worth of crossovers. But, with Spiderman getting an Arena Universes Within, it had me thinking- Since I like making proxies in my own downtime, it's a fun thought experiment, what Universes Within would you want to see? What plane(s) would they be set on?

For me, two come to mind: Lord Of The Rings, and Avatar: The Last Airbender. Don't get me wrong, love both these sets as is, but sometimes I don't want to play with cards that come from preexisting IP (even if it's one I love to bits), especially with the drama surrounding a certain Toph print.

For ATLA, the "where" is easy: Zendikar! It's already a plane heavy in Mana, where the land literally comes to life. I even have names for the bending mechanics in mind (Vanish, Invoke, Landrise, and Erupt for Air, Water, Earth, and Firebending, respectively).

For LotR though, things get tricky. There isn't one plane that has all of Elves, Orcs, and Halflings all in one world except the D&D worlds (at least, in terms of what WotC owns), however we could say it's a matter of Omenpaths opening and multiple factions engaging in warfare. The Orcs could be Ixilanian, Halflings could probably work as Kithkin from Lorwyn (though, I admit that's a bit stretching it), but I don't know where I'd put the Elves to be- Simic Elves aren't super common outside of Ravnica, but a couple exist in a handful of one-off examples in other planes.