I seem to do this a lot - I’ll find a game whose setting I really like, but dislike the ruleset it uses, so I end up converting it over to the ruleset I prefer (usually Cypher System). I’ve learned that to some people, this is blasphemy of the highest order, which is surprising. I still do it, though. 😬
I used nWoD for fantasy games. The core dice pool system works pretty well, and few things are tightly coupled to any setting in particular.
I mostly don’t like d20, so when someone tells me they’re doing a game about secret vampire societies in it, I’m a lot more disappointed than if they ported a system I like or am neutral on. Also 5th edition in particular makes a lot of assumptions about how things work.
This. NWod/ChoD system just works so well. Of course I’d avoid it if the setting does not fit, but non-thematic d20 should be replaced by dice pool whenever one can do it. I haven’t tried this yet but I really want to run a medieval fantasy game with Mage magic system. Pretty sure there are some games already implementing something like that from the start, but if magic was ever a thing then mage seems like the way it would be approached, messy and dangerous.
I love mage, but its magic is so involved and powerful I wouldn’t want to use it in a game unless everyone was a mage. You don’t really want one players options to be “I hit him with my club” and the others to be boundless.
Awakening 2nd edition was really good, imo, but I never actually got to play Ascension