High Level Slaad - The lower challenges are adequately covered and the death slaad comes in at challenge 10, so there needs to be a few more varieties at the mid and top end of the challenge spectrum.
Fey - I'm running a campaign now with a bit of a focus on fey and there is an absolute woeful dearth of decent fey creatures. Even when going through my old Pathfinder and 3e and 2e sources, I really have to dig to find any sometimes. Some mid challenge and a few high challenge opponents would be great. Also, the quickling. I love those guys.
Demodands - Why not? There are only three of them.
Annis Hags - Actually, maybe even add some new varieties of hags as well
More Varieties of Trolls - Two-Headed, Frost, Giant, Spirit, and Spectral
More Dire Creatures - Especially bears and weasels!
Some more mid-level undead - From like challenge 6 up to challenge 13, there are no undead. Things like the bodak, devourer, nightshades, crimson death, boneclaws, and the like. Even some more varieties of skeletons and zombies might work too.
More Beetles - I don't know why, but I love throwing beetles at my players. I also like using them in encounters.
Some Missing 'Classics' - Spawn of Kyuss, Frost Wurms, Sword Spiders, Derro, Greater Basilisks, Dracolisks, Deepspawn, Gibberlings, Leucrotta, Skeleton Warriors
No Dragons - They take up a lot of space and the MM already covers a bunch of them. Maybe some drakes or dragon-like critters instead?
No More Giants - Too many giants are mechanically similar bags of hit points with big weapons. No reason to bring back or create new varieties unless they're going to be different and interesting both mechanically and flavour-wise. And no 'it lives in the jungle instead of in hills and has 50 more hit points' is not interesting to me.