The most famous sequel in GD history was Bloodbath, but it doesn't look anything like Cataclysm that came before it. It probably wasn't even designed as a sequel either, but it fulfills that purpose much better than most GD sequels have in my opinion. A lot of early level series got the whole sequel idea right too. The Fusion series, Sonic series, Artificial Ascent's series, Plasma Pulse series, and even both of the Killbot levels approach sequels in a way much more enjoyable than simply outputting the same level structure but different.
I'm mostly making this post because of POW since that recently got finished, and a quick rundown on my opinion is that it tries too hard to match the original structure of the level when the song clearly doesn't fit in some areas, and cutting the song removes good opportunities to add flare to stand it out from WOW more than just a color scheme swap. A lot of others that feel like they try to mirror their older levels too hard sort of apply.

A big example of a level that doesn't work as a sequel personally is Hybrid's Deimos. It just seems sort of pretentious in how it falls over backward trying to show how it resembles Phobos, most notably how Smokes' fades his part with similar structuring to Phobos' palette as if you needed to be reminded that Deimos was a sequel. Tygresek/Juhou's part also is just rainbow because that's what Ty's part was in the first level, even though what made his part memorable in that level was that it marked a shift in the song and emulated the sort of trippy tones that came in. The entire level is also made using tons of 1.9 features, as if that was the only thing that made Phobos memorable and just isn't used to capture the essence that Phobos had. Hybrid's Deimos isn't a bad level, though, it just feels more like a loose remake than what a sequel should strive to be. Skele's approach to the level sequel is what I think needs to be emulated more, where you try to emulate the feeling and atmosphere of the original in an updated way without shoving assets of the original in your face where it doesn't actually enhance the level.

That's not to say to just completely ignore the first level and make something that doesn't resemble it at all. This video showcases 3 concepts of Cosmic Cyclone, which Cyclic officially called a sequel in his part's description. The third one being what we got just doesn't work as a sequel of the original. It looks like a sequel to the released version of Sonic Wave Infinity which did not exist when the layout was made. Regardless of quality opinions, I feel like Cirtrax and Ossy at the very least approached decorating the level in a better way where they actually used a nine circles effect and had vanilla blocks rather than making it look nothing like SW. Now I'm not entirely sure if APTeam thought of Cosmic Cyclone as a sequel in the first place, but that is what the level was intended to be when the layout was made and finished, and I wish to make a statement regarding how decoration alone can make or break a sequel.

Overall, I think many sequels border emulation rather than actually "Following Up" and being allowed to present new ideas. Killbot's sequel completely throws the color scheme, even the original "computer virus" theme away to develop the concept into something new. Requiem allows itself to establish a separate identity from the original, watching it blind w/o prior knowledge you probably wouldn't even realize it was a sequel until 9% when the font and aggressive screen blockage make its appearance. Requiem's approach just allows for more expression and produces more memorable levels, something that can stand alone while still following up the original.

Sort of long post so hopefully I didn't lose anyone but what do y'all think?