I'll be exploring the fringe giants too (death, fog, sand, sea, jungle, etc.).
Missed this on the giants earlier, but I'm glad to hear it. Since verbeeg were featured in
Rime of the Frostmaiden, could we have some more variants for them as well? (Edit, forgot they were in MME2, but more variants would be fine) And voadkyn as well, although they are rather obscure.
Another classic creature that's only appeared in an adventure so far (
Out of the Abyss) is the ixitxachitl. The vampiric version appears in that book as well, but it would be nice to have further variants.
I'd like to see some higher CR oozes, but that might be difficult other than making them just huge. Also, the mustard jelly is a classic ooze which has yet to be updated.
It would be nice to have further fey, depending on what
Witchlight has in its bestiary. It's actually fairly surprising that the spriggan hasn't been updated for 5e yet. The sirine has been sort of updated (as a singular creature named "Siren" in the updated Tomb of Horrors), but it would be nice to have it fully updated and with variants. Other classic fey like the leprechaun or brownie would be nice as well.
There are a few dinosaur/Mesozoic reptile niches which could be filled. Most notably 5e is missing
Ichthyosaurus, which is the third of the classic Mesozoic marine reptiles (MM has the
Plesiosaurus and MME1 has the
Mosasaurus) that (I believe) have appeared in every edition. A smaller pterosaur like
Pterodactylus would be nice. As for actual dinosaurs,
Iguanodon has appeared in previous editions and its thumb spikes make for unique natural weapons. A predator like
Gorgosaurus* or
Albertosaurus would fill the predator CR gap between
Allosaurus (CR 2) and
Tyrannosaurus rex (CR 8). And it would be nice to have one of the really huge sauropods like
Patagotitan or
Argentosaurus that dwarfed even a
Brontosaurus! (Sorry, I love dinos in D&D, so I'm probably getting carried away!)
Again, that's all I can think of for now, but I'll probably come up with more in the future!
*
Gorgosaurus was definitely in previous editions - a very, very memorable game session back in 2e featured one that somehow sneaked up on us on a "flat treeless plain", scaring off our horses that were unfortunately tied to a petrified character (as there were no trees to tie them to); the petrified character's player was hugely annoyed we had done that, and we had fun hunting down all his scattered stone parts after dispatching the beast!