Wizards of the Coast's just-announced Magic: The Gathering brand for non-WotC properties, Universes Beyond, will be legal for sanctioned play in the game's eternal formats according to Wizards Play Network Community Manager Nelson Brown.
As stated in a Facebook comment by Brown, "...you can count on these to work similarly to other non-Standard releases: legal in eternal formats unless otherwise specified."
In essence, that means that these third-party franchise themed Magic cards (the company has so far confirmed Lord of the Rings and the science fiction genre Warhammer 40,000) will be allowed for play in Legacy, Vintage, Commander/EDH, Oathbreaker, and Pauper.
"This will make legacy feel so cheesey... Rick Grimes, Steadfast Leader was bad enough for me to take apart my legacy humans deck and stop buying my usual 4-6 boxes per set," says Reddit user MadMonsterSlayer. "I kept playing, but even though I don't mind Lord of the Rings or Warhammer, I fear Universes Beyond will take the game to a place where I can't choose to run from cards like Rick."
Another Redditor, pso_lemon, shares their thoughts on Universes Beyond in Magic: The Gathering's eternal formats, stating "This news is worrying to say the least. It might be time for me to pick up a new hobby and leave this to the next generation. I don't know, time will tell."
Whether or not these cards will ultimately be allowed in Magic: The Gathering's eternal formats once they become a physical reality is up to Wizards of the Coast, but for those who truly do not want Universes Beyond cards mixed in with traditional Magic cards, there is hope. The Commander and Oathbreaker formats each have their own rules committees that are separate from Wizards of the Coast. There is the possibility that either, or both, may decide do disallow the Universes Beyond cards from their respective formats.
Otherwise, eternal players might need to get used to their opponents tapping their Sol Ring to equip their Frodo Baggins with a plasma blastgun.