I am a large fan of this mod, and a huge fan of Lovecraft's work. What drew me to this mod in the first place was the fact that AbyssalCraft is the only Minecraft mod adding any of Lovecraft's work to the game. Seeing as most of Lovecraft's work takes place on the Earth, most of my suggestions will be focused in the Overworld. That works pretty well with the exploration focus of the mod, I think. I will also be explaining the details of the models and reasons because I don't know and won't assume how familiar you are with Lovecraft's work.

Deep Ones--- I think the Deep One could be a great addition to the beaches and swamps of the Overworld. They would have the classic fish-man look and their model could be a player model with a Deep One skin added to it (to keep things a bit simple) or add some fin spines the the head. They could have an uncommon temple-like building made from a greenish stone in the deep ocean biomes under the water that spawns a bunch of them as well as them being a uncommon spawn on beaches. In the Mythos they can be on land or underwater, but they should definitely have water breathing so they can be deep one.

Elder Things --- The Elder Things are another great mob that could go in the world. Spawning in the cold biomes, especially the cold mountains biome, they could have dungeon-like towers or complexes where there would be Elder Things as well as Shoggoths, seeing as the Elder things made the Shoggoths as slaves. The Shoggoths and Elder Things would be passive towards each other. The loot could be lots of redstone/high tech stuff becuase of them being an advanced race. The model could be a tall (2 or 3 blocks high) purple creature with a set of wings. (They can't fly, though. They evolved out of the wings.) The building could be made out of a white stone brick (possible called snow bricks), snow, and ice.

Mi-go --- Mi-go are like a gross cross between a fungus and a lobster, with batwings that let them fly and usually lots of eye-stalks. They could spawn flying above swamps and forests.

Bosses --- Any of the Great Old Ones or Elder Gods could be bosses. Their spawning could be biome specific (Cthulhu in the Ocean, with a one of a kind, like the End Stronghold, unique structure called Rlyeh) or through a ritual (summoning Nyarlathotep with a sacrifice of some kind and having a complete book, like the vampirism book in Witchery) or in another dimension (Yog-Sothoth in an End-like dimension with loads of portal-looking things or a crazy nether portal animation skybox. The portal part because Yog-Sothoth is the keeper of the gates, or portals, to other dimensions.) There are so many to choose from. The real challenge would be to make them an impressive size. Most of the Old Ones or Elder Gods are massive.

I love these ideas... but I disagree with the Great Old Ones and Elder Gods being bosses. A big part of the Lovecraft Elder Gods is that you simply do not stand a change when fighting them. I think having them as bosses would be inappropriate.

I know J'zahar is a boss, but even he is a mere simulacrum. Plus, he does not seem to be all that serious about killing the player.

More bosses would be nice, but i feel like j'zahar and the other bosses fit more with the scale.
i mean the dreadlands boss is just some foreman and j'zahar himself is just a inter-dimensional HR guy.