I've got a couple of thought here, leading to two very different conclusions.
The first, is that Vistani will be revisited with a whole selection of "planar racial variants" in a Planescape book. When I say "Planescape Book" I don't necessarily mean a setting book, just a book that is themed around the planes, and adds a bunch of new player options, racial options, monster statblocks, etc. This aligns with a bunch of tidbits we've gotten, including the many UA player options being tested, psionics, and gem dragons. Vistani are of course tied usually to Ravenloft and the Shadowfel, so they would fit in such a book as another racial option.
The second idea, and this is a little more remote, is that Kate Welch once teased how she was developing a book with many authors from various corners submitting their own adventures; these authors included those from the DMsGuild, but also had Marisha Ray and Deborah Ann Woll. I haven't heard any hints since to this product so it could well be dead, but I find it more likely it was simply postponed to fill the next available "release timeslot" (as we know 5E loathes rushing things, and there were clearly many books like Wildemount/Theros that needed a release date). Perkins also gave a hint that if you like Curse of Strahd, you'll love what he's working on. That Perkins hint could just be for Rime of the Frostmaiden (has a horror theme) but it would be interesting if this "Adventure Compilation" is specifically a compilation of horror adventures, set in the Domains of Dread where Vistani have a role.
These are just theories of course, and could both be completely off base.
Also noticed that both Descent in Avernus and now Rime of the Frostmaiden re-use material printed for D&D Next (Murder in Baldur's Gate and Legacy of the Crystal Shard respectively). It would be interesting if next year the adventure reuses material from the last D&D Next adventure, Dreams of the Red Wizards. Part of it already got reprinted in Tales of the Yawning Portal so I doubt this personally, but actually having an adventure in Thay would be pretty dope.