Small multiverses in fiction.

[Rynjin]

I think OP's point is that the multiverses follow different rules?
That's why they are multiverses as opposed to "planets of hats", but maybe the distinction is only semantic...

Actually, speaking of the different levels of magic in each multiverse, there's the manga series which does that: Tsubasa: Reservoir Chronicle
Some multiverses are entirely based on magic, others on technology, etc... and the characters have to follow certain rules in each of them.

Sadly, the anime adaptation it got veered very well off the intended plotline.
Like, it's straight up just a different story past a certain (quite early) point.
There are some decently high-quality OVAs (direct-to-video releases) but they are nigh incomprehensible without familiarity with the source.
And they only cover maybe like 1% of the story.

I have seen it sold here occasionally.
However, given how old it is, idk if many official translators still stock/sell it abroad.
but I would never advocate you "sail the grand line"...

Tsubasa is good fun, but you get into the issue of not just having to read that, but also xxxHolic (it's not porn I swear) and swap between them at relatively hard to determine times to get the full plot...which kinda goes off the rails about 2/3 to 3/4 of the way through both of them anyway.