Ironically part of what makes it niche is how dedicated its fanbase is. It seems there are no casual American anime fans, unlike say Star Wars or MCU. And to show it's not a generational thing, my closest analogy would be Gen X/early Millennials with Dr. Who. Clearly niche -- it aired on PBS then BBC America -- but a subset of American nerds obsessed over it. Was a perpetual wish list item for a theme park attraction or at least an HHN house. That said, while there may have been a Fathom events screening or two, never been a major motion picture. Which is probably the bare minimum to be included in UOR.
Yes, Dr. Who is a good example as well
This brings up another important point for a theme park installation
The property needs to be multi-generational or, forgive the pun, universal in archetype
For instance, my family doesn't know a lot about Attack on Titan or Demon Slayer and I doubt they would ever watch it
But you can be sure if I ask them who Mario is, they'd be able to tell me
Everyone knows he's a hit R&B music sensation with hits like How Do I Breathe and Let Me Love You
And then there is the inevitable HTTYD example, which is really just an extension of the universal "dragon" and "viking" archetype...
Although, the property itself are able to be enjoyed by a multi-generational audience
I'm, again, not sure what anime property checks those boxes