More random former core monsters that come to mind:
Achaierai (3.0/3.5 Monster Manual)
I don't know why I like a monster that's basically an evil giant bird head on stilts, but I do. If we get more adventures set in the Lower Planes, I hope it makes at least a cameo.
Fremlin (Monstrous Manual)
Tiny winged gremlin relatives with a side of ugly cute. And also a PC option in 2E! Suppose you could reskin the new Fairy for that purpose in 5E.
Immolith (4E Monster Manual)
What happens when a bunch of demons come back wrong. A nice use of 4E's demon lore, tapping into that elemental origin. I still think it could be easily fit into 5E.
Masher (1E Monster Manual)
Another forgotten aquatic monster. Objectively, nothing too special, but I guess I liked their name more than anything else.
Mudman (MC Vol. 2, Monstrous Manual)
A simple monster concept - magically animated mud creatures - that I'm surprised hasn't returned in some form in 5E.
Nixie (Original boxed set, Basic 1977, D&D Rules Cyclopedia, 1E Monster Manual, Monstrous Manual, 3.0/3.5 Monster Manual)
Not a monster I have a strong personal attachment to, but considering how the nixie was core in nearly every edition, and it's a simple concept (sea fairy), it's surprising they haven't made a comeback.
Nuckalavee (D&D Rules Cyclopedia)
A really bizarre creature that seems like it would be a great fey foe in 5E. (There is
an unofficial version in 5E thanks to the Tome of Horrors.)
Yrthak (3.0/3.5 Monster Manual)
One of the more interesting niche-filling monsters from 3E to me, probably because it seems very kaiju.