Certainly this game seems to me more true to Lovecraft stories than others, although this may only be a personal opinion. I found the game to run better, and more like a Lovecraft story than Arkham Horror and Elder Sign, (although don't mistake me, they are good games). The four Ancient Ones chosen for the original games are the obvious four that pose the greatest threat to mankind. In fact I was rather surprised to see Althach Nacha as an epic monster, because it is said, (I can't remember where), that when its web is finished the end of mankind has come.

If Fantasy Flight Games are releasing supplements introducing a new Ancient One as well as more mysteries and perhaps more epic monsters, the examples I thought of are these.

Ancient Ones

Nyarlathotep. Really he should be the first, because of the extent of his influence around the globe and that he would like to see mankind fall into chaos and madness by what he has given him. His Reckoning would be the placement of a random Mask as an epic monster, (e.g. the Bloody Tongue in the Heart of Africa, the Bloated Woman in Shanghai, the Black Pharoah at The Pyramids etc.) There would be a means of banishing the Mask rather than fighting him. If a Mask already exists, it is removed but Doom is advanced one place. If a Mask is banished then doom retreats one. His mysteries coud involve such things as the attempt to reinstate the worship of Nephren-Ka in Eygpt, the Chandler Foundation trying to take over America, and the Skinless One trying to exert his influence in Europe.

Abhoth. Although this creature physically manifests in America, the bulk of its body is in another dimension. Its Reckoning could be the appearance of spawn, and if enough appear, doom advances. Its mysteries could include the disappearance of communities lures by its illusory paradise, or an expedition to discover Atlantis from where clues could be discovered to drive away the god.

Hastur. Although a Great Old One rather than a god, Hastur has considerable influence on Earth due to The King in Yellow and the activities of his servitors, the byakhee. His Reckoning would be the effect of the King in Yellow across the world, perhaps allies are lost because they are driven mad or a town cannot be entered, unless enough Eldritch tokens were placed on, or removed from, his card. His mysterious could deal with the effect of the King in Yellow in a location, the appearance of The Palid Mask that must be found through an expedition, the appearance of the Hunchbacked Gentleman (Hastur's Harbinger) as an epic monster, or a mystery that draws the investigators to Lake Hali and Lost Carcosa.

Other possibilities, (simply because of their size and destructive power), could be Cthugha, Shudd M'ell, Ghatanothoa (especially with his link to lloigor) and Zhar.

Epic Monsters

Yig in America, perhaps as a rumor, causing the appearance of serpent people until stopped.

Glaaki in London.

Chaugnar Faugn in the Himilayas.

Zoth-Ommog somewhere in the Pacific.

Ubbo-Sathla in Antarctica, perhaps as a rumor, (although immobile and unintelligent it may be that it is ready for a convulsion that would tear apart the planet).

Tsathoggua. (Because he lives in N'Kai he could pop up just about anywhere).

And of course, these are only a few of what's available.