I don't think there are any cards I won't use, per se, but I won't use a card just because it's really good, unless it has some greater synergy with my deck. For instance, I wouldn't run
Cyclonic Rift in a bantchantress deck, but I'd absolutely
consider it for a spell slinger deck. Even then, if I'm trying to combo off, I still would probably cut it, but if I was trying to win off of prowess triggers and combat, I'd keep it in. It has to be not just good, but good for my deck.
I also want to comment on the universes beyond thing, but not to say they're all out. MTG has always been a "universes beyond" style game, whether they had a name for it or not. One of the first sets in was Arabian Nights, and right at the beginning of the modern era we had the Kamigawa block, and then before the infamous
Walking Dead Secret Lair, we'd just had Throne of Eldraine. Each pulls cards right from actual, real-world stories that aren't in the MTG cannon. There are dozens of examples of Magic doing this over the years, but whether it's Greek Mythology, Gothic Horror, or a Fairy Tale, all the tie ins up to that point had three things in common: they were set in a land without modern (or futuristic) technology, they all had some sort of magic at the core of their stories, and they all involved fantastic beasts and creatures. I imagine, since it meets the criteria, the upcoming LotR sets will fit in a little more nicely with the established game, but things like Dr. Who and Warhammer, will still feel out of place, though, I , personally, am about playing the game more than the flavor of specific cards. It's never made sense for a goat token to drive a plane and then to get blocked by a big spider.