I'll do my bit in mentioning it to my friends. I've already introduced three people, here's hoping for the best. As for the lore, well, I don't see a reason why you couldn't just start the game with a "select your Ancient Evil" screen, which changes what particular nasty is imprisoned beneath the ground/exists only as a shadowy wraith/etc? If you need ideas, well, you could always look into the Cthulhu Mythos, or if you want to take the short route, you could take a quick look at Arkham Horror (which draws so heavily from Lovecraft that it actually uses lots of his characters directly). One thing you might leverage is the fact that fishmen have a long and storied history in cosmic horror, since in Lovecraft they were basically Cthulhu's parahuman cultist "children". Just a few ideas:
*A sea deity/spirit/giant cosmic squid that gets a bonus to spawning and using Fishmen.
*A necromantic being that draws strength from death - perhaps stronger plagues/undead?
*A sort of Primal Entity that has influence over natural forces, like volcanoes and earthquakes.
*A Planetary Parasite that spreads a unique type of terrain that is useful only to itself and its minions, while being hostile to all other life.
All of this is just spitballing, of course. No guarantee that any of these are suitable for the actual game.