The only megathread I can particular think of (apologies if I have forgotten anyone) would be the Jinyu/Ankoan one by Rorrand, who I think is still around on the forums:
It hasn’t had too much activity lately, but still worth mentioning.
Gnomest is in the process of making a new Gilblin megathread, since the OP of the last one has stopped posting. I’ll make sure to mention it when it gets posted.
There was originally a combo Arakkoa/Ogre megathread that was replaced by someone else with a full-on Arakkoa megathread, but the OP had to quit playing the game and hasn’t returned in awhile. It has sadly been inactive for awhile.
Other than that, I can’t think of any other megathreads (again, apologies if I forgot anyone!) I’ve seen some pop up awhile ago and sadly fizzle out quickly. It’s not easy to set up a megathread, let alone keeping up support for it.
There was kinda a Saberon megathread, but it wasn’t named as such and died off some time ago. I see people mentioning wanting Saberon here and there, and I actually would like Saberon playable myself. If you ever decide to go for it, I’ll be sure to support it. 
I’ll be sure to link my new Ogre megathread when I can get it up and running. Honestly, Ogres and Saurok are the two races I could passionately do the work for and maintain megathreads for. I don’t think I could handle more than that, and it wouldn’t do other races I want to an extent any justice to spread myself thin.
It’s good to see people working together in their race requests. It’s what I like to see around here, as arguing about taking up slots, being viewed as competing with others, and getting yelled at for wanting a particular race playable is no fun at all. I prefer to support each other and celebrate when we get new races people really want.
Anyway, with any luck, people will get the races they want playable and we’ll all get to play with each other as them. 