I absolutely love Warhammer and LotR, and I mentioned in a previous post that something like Dune would lend itself to set design pretty easily. However, I really don't like the idea of black bordered magic cards with Adeptus Astartes marines, Gandalf, and Shai-Hulud all in the same deck.
Magic is great because the different planes always manage to feel like they belong in the greater scope of the game's story (by this I mean less the actual story novels and more
the general art/themes in magic), but adding new, highly recognizable, and disparate franchises sounds like the game will lose that sense of cohesion. If magic introduced technology worshipping superhumans in
power armor (I was going to say wizards and giant worms, but we already have wizards and wurms), it would feel like another plane that could interact with another part of magic. Like how any plane can suffer from a Phyrexian invasion or a sudden appearance of the
Avengers superfriends.
Elesh Norn showing up in Ravnica or Zendikar makes sense because that character's story develops with the idea that those planes exist, even if the development team didn't have concrete ideas of eventually fitting them together. If Aragorn found himself in Innistrad, his first thought would probably not be "Hey, let's fight all the vampires and other scary things with the other humans." His character just doesn't fit. He wasn't written with the abilities of magic's other characters or the whole multiplanar setting in mind.
I feel like I'm ranting at this point, so tldr: booo.