Sparkdog on r/DnD posted this list:
https://www.reddit.com/r/DnD/comments/5mcos3/potential_new_monster_stats_from_tales_from_the/
Some of these, like the ogre mage, are already updated, as was pointed out to him in the responses.
But... if we get all that, that would be a bestiary at least 1/3 to 1/2 the size we got in VGtM. Honestly, I can't see the monster section in the book being less than 20 pages (which, given 30 pages for each of the seven dungeons, comes out to near the page count for the book), which, if this is the case, will definitely ease any questions I have on purchasing it!
Monsters I find notable:
Elemental grues and thoqqua - it would be very nice to have them updated, as they are good lower-level elemental encounters.
White pudding and elder black pudding - we definitely need more oozes, and these two would definitely fit the bill.
Pyrohydra - I was a bit saddened when we didn't see these in the MM as a hydra variant. But, of course, limited space... Hopefully we'll see their cousins the cryohydra as well...
Kelpie and Nereid - More fey, and water-based ones at that. The nereid may need some modernizing however.
Giant and drow variants - always nice to have those, although, as I said, it may make some of my own work on giant variants obsolete.
Looking at this, some of what was in and not in VGtM make a bit more sense. You have to wonder if
Expedition to the Barrier Peaks was going to be in this collection at some early planning stages, but was later dropped; but since the froghemoth and vegepygmies were already updated, they decided to put them in VGtM instead. And it might explain why the giant variants in VGtM are a bit esoteric, if they planned to have more conventional giant elites updated in TftYP....