I mean, I liberally plumb other systems for content in general, so relatively all the time. Mechanics, setting, anything I can pull that sounds good to me.

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.

My first "Shadowrun" game was just taking the setting and using D20 Modern for the rules since I didn't have the actual Shadowrun books (and couldn't find them for sale either).

Sometimes, after playing actual Shadowrun, I feel like the D20 Modern rules worked better. 🤣

Blades in the Dark loadout mechanic I've used in almost every one shot regardless of system. Too useful

Not what you are talking about but one reason I learned to love the champions system from the early 90's was a guy there ran a fantasy setting using it. He had a program to he made himself to do the heavy lifting but the variable power pool rules gave him a great ability to have characters have any spell they wanted by talking to him. Just had to fit within the pool which as a magic user would level as points were put in.

That depends. Sometimes the mechanics are kind of integral part of the setting. Those reflect some realities of the setting. But then, if the mechanics are too crunchy for me, I would be looking for "X with serial numbers filed off" hack of another system

Greyhawk is far and away my preferred setting for D&D-like games, I've run a bunch of rules in it but mostly 13th Age and Shadowdark.

This is the secret to being a dm and "having so many cool unique ideas"

Never.

100% done this. Some systems just gel with me better (like nWoD or CofD). I've always wanted to try this with Curse of Strahd.