This isn't really my scene, but the Link to the Past // Super Metroid randomiser[1], where you keep switching between the two games, potentially discovering items from one in the other, is something that is impressive that it's even technically possible.

[1] https://kotaku.com/inside-the-wild-new-mash-up-of-link-to-th...

Some of the more well known ones are Final Fantasy, A Link to the Past, and Pokemon.

Here is a comprehensive list of games that currently have randomizers written for them.

Full disclosure, I've written two of the Kirby randomizers ;)