Ground Zero was one of those I pondered doing a sequel for before, but just never came up with something that I thought would follow it up properly. If I was going to pick a canon ending for GZ, it probably would be the 3rd one (Atomic Alliance ending) since it offers a little more flexibility as far as world building and still keeps the protagonist from GZ relevant, even if Vincent is long dead by that point, his descendent(s) can carry on. (And yes, that's the GZ protagonist, you just learned another nameless protagonist name)
Most of Suzy’s endings sort of follow up a bit on GZ, but only very briefly. (And certainly not to the extent of the GZ epilogues)
Alpha Wolf is one of those self contained stories that I don’t see me ever really revisiting again. Though one never knows. I always see that one that doesn’t quite fit any of the established story “universes” mainly because it’s more like an alternate earth where the movie monsters were real. Even with some of the weirdness going on in the “Suzyverse”, movie monsters being the most normal thing ever isn’t common in the modern setting.
As a general rule I used to shy away from sequels mainly because I always felt most of the stories I did were “told” so there wasn’t really a need or desire to follow up on them plus I never wanted to ruin the original with a cheap sequel.
I’m a little more open to sequel ideas of course now, though I still am hesitant to do them unless I got a really good idea for a follow up and even then, it’s typically not a traditional sequel with the same character, just the same world (Like Innkeeper and Rogues)
Expanding the Necromancer lore/stuff is something I’ve gone back and forth on a few times, but as I said, it’s one of those that I’d really need to solidify before doing it. Hence why I’ve never really done a proper sequel for him and just have him pop up only at key times in other stories just as a reminder that he did establish a place for himself in the multiverse as it were.
Eternal was one of those where I really did feel like the story was “done.” I had no plans in making anything else for that world. The best way to do this is to kill the world of course. Lol. Eternal Bliss was always my head canon anyway, though I didn’t really make that ending canon until I decided to bring back Semra.
Of course I had to figure out how transport her to a new world so she could be awful to people there in the future, so the ideas for the Vile story followed.
As for Tanya and Aron, they’re a bit something of a star crossed couple and have other pressing concerns at the moment (Tanya especially) but again who knows what the future holds.