I cant imagine a new race, but for balance I can see existing races made playable but with conditions, like an evolutionary(progenary?) choice made by dinosaurs when presented with the option by a Deity.
Conscious dinosaur deific choice. Deity: "I'm going to change things around, this form? *waves open palm in small slow circles at dinosaur-kind* is no good and will have to change. The thing I started is evolving nicely, except you guys. What do you want?" Conscious dinosaur: We want to fly! deity: *jaw drops* ...fine... but this will be a drastic change, you'll be a heck of a lot smaller- I mean wayyy smaller and lighter, obviously, same brain size heh,but brain maybe a bit larger proportionally by comparison so there's that, and you wont be the top of the food chain anymore you will be easy to kill if caught but...you can fly. I'll adjust vision as well for the height differential. Conscious dinosaur: "we'll take it!" *to themselves- we can fly!*
dudimous said: As a Cthulhu Mythos fan, the concept of long lost and forgotten civilizations pings with me
It didn't occur to me when I first read the OP, but once you mentioned it I went back and yes indeed, I got Cthulhu feelin' from the description. Good catch. (it has been decades since I read any Lovecraft)