There are plenty of 'great old one'-type creatures from Lovecraft that haven't been used yet, and waaaay more once you factor in Clark Ashton Smith, Lin Carter, even Brian Lumley and about twenty other Mythos writers.

My guess for the GOOs in IH would be something like: Dagon, Mother Hydra, Bokrug, and Ubbo-Sathla. That satisfies the emphasis on 'core' Mythos, and ticks the boxes for things which are pretty much 'gods', things which are ambiguously giant monsters, and things which are just shapeless incomprehensible protoplasms with weird hyphenated names.

Mother Hydra and Father Dagon are probably going to turn out to be Heralds, I think. Some of the things in Arkham Horror are adaptations from The Call of Cthulhu RPG, in which Dagon and Hydra do not count as ancient ones. Also, given that mighty Ghroth and Tulzscha have been reduced to Heralds, I doubt that the meek uber-Deep Ones will become ancient ones.

That being said, the formula for expansions so far has been: 2 Clark Ashton Smith Deities, 1 Brian Lumley, 1 Ramsey Campbell (Dunwich Horror), with one less deity for Smith and an additional for Campbell in Kingsport Horror. One would reckon that they'd go for 1 of each Smith and Campbell and 2 of Lumley this time around, but given that Bokrug might make an appearance somehow (he could be a Herald, or like tha Tarot card set, reduced to his statuary artifact status) and that 2 examples is a bit few to make a solid case, this might turn out to be totally different.

If it does however turn out to be 1 smith, 1 Campbell and 2 Lumley, my likelihood meter tells me that the following might be a possible team of ancient ones: Ubbo-Sathla, Daoloth, Othuum, Bugg-Shash. The Lumley dieities (Bugg-Shash and Othuum) give me a warning bell though, as they are a bit less known than the deities used so far, which have all been either very well known Lovecraft entities or well known entities from the three already mentioned authors and August Derleth. If Bokrug is used in some fashion, I hope that the two other Lovecraft entities, Rhan-Tegoth and Ghatanothoa, also return in some fashion. Perhaps instead of Bugg-Shash and Othuum we will see Ghatanothoa and some derlethian critter (hopefully Zhar+Lloigor as a singleancient one card), with Rhan-Tegoth and Bokrug filling possible Herald slots (there were 2 in the kingsport expansion, maybe there will be two here as well), in which case I wouldn't mind Dagon and Hydra simply being brutal unique monsters (since you can fight star spawn and the effin' Sphinx, those two shouldn't be a problem).

If two more guardians make their appearance, they would probably be Lumley's Kthanid and Yad-Thaddag. Yad-Thaddag is not very well known though.

Useless speculation is fun.