Zelda 2 and Metroid 2 as the sequels that never got their own sequels. I like to think about an alternate universe where Zelda 2 and Metroid 2 became the canon version of the games that always get imitated rather than their SNES counterparts.
I think playing both side-by-side is a really interesting study in how experimental Nintendo used to be with their own IP, and also really drills down on how unexperimental they were essentially from the SNES to the end of the Gamecube era.
Actually this is kind of a trilogy - Wario Land is the Weird Mario Sequel, but they actually ran with that for 4 games instead of just dumping most of the ideas. The main difference being that it didn’t star the actual star of the previous games, so it could have its own little branch without interfering with the rest of the series.