Nintendo is pretty litigious and protective of their IP. A "Zelda MMO" would get a cease-and-desist letter that would make the authors' heads spin so quickly they'd travel back in time where they'd be dead.
Some fans are tired at the lack of innovation from the IP holders and are taking fan games in interesting directions. Look at this for example:
https://undreamedpanic.itch.io/gamma-emerald-ea
Loads more here:
https://www.ign.com/articles/zelda-fan-ditches-ocarina-of-ti...
I think we're going to see real quick how this is going to go with those Adobe clones. Clean-room is supposed to be legal. I have a feeling it's about to stop being legal.
But then that will make PC-clones illegal: meaning all modern computers. It's a curious problem for them.
If you clean room reverse it, the mechanics in a Legend of Zelda game are, for the most part I think in the clear legally speaking. But part of what makes Zelda feel like Zelda (IMHO) is that you’re fighting familiar monsters in a familiar setting with familiar music aka all the lore, otherwise its you've just rebuilt an open world exploration game with dungeons.
However if you borrow lore like even if you change the names of all the monsters, you may still run into legal risk if they remain visibly recognizable. Even if you redraw them in a different style, if they’re still identifiable as Octoroks/Moblins/etc, that might still open you up to litigation.
There was a similar case a while ago, Tetris v. Xio, which might be worth looking into, since I think it sets some useful precedent.
https://en.wikipedia.org/wiki/Tetris_Holding,_LLC_v._Xio_Int....
But to be honest - I just want an open world MMO. I don't need Octoroks, etc.
In fact..... for the last few weeks I've been working on this sort of thing for Animal Crossing. I replaced all the assets with little felt toys. (meshy.ai makes fantastic 3D meshes). But the code only got halfway done before the onslaught of stuff hit this last week... so I lost motivation. Also the clean-room thing is really expensive.