While such ideas are interesting, I'm not wild about an entire hostile civilization with abilities like telekinesis or thunderbolts since a full siege of them might be impossible to defeat and I've always liked DF's approach to fantasy where those sorts of powers are highly uncommon. I would not be opposed to such powers appearing on megabeasts, so maybe I could make creatures like a Royal Alicorn, Elder Thunderbird or Kyuubi-Oh that are completely animal-like and appear as megabeasts. It would be interesting if they can appear as allies of their associated civs. I think I've read plans to make civs that can elect a dragon as their leader, but it's a little complicated.
I might have forgot to explain this properly, the idea for these civs is that they are still anthros, not monster civs like the wyverns, and they will still be a copy of the original culture (same clothes/weapons/ethics), just with a new creature and the item thief/baby snatcher tags. They will replace the badified copy civs that are in play now.
I thought about Qilin as the badified version of pandamons, but I felt like bears and foo-dogs are more familiar and I guess the Qilin does sound kungfu-ish enough to me and most people know it as the Kirin, which is japanese which would make it a weird choice to adopt the china culture. Also, qilin is the chinese word for a giraffe, which makes things even weirder. I'm getting more comfortable with the idea of Foo-dogs actually. Since most of these creatures are larger versions of the originals, it would be interesting to have one that gets smaller.
Bearzerkers are also a potential badification of Haukyngrs, since they are Norse. If I wanted to put Thunderbirds on a different culture, they might work as a badification of Mammovichs, since they are vaguely arctic american themed. I do like the name "Grootslang" but I could make that a normal animal or megabeast instead.
Venom could work for Myrmidons. As you've noted, Formecians aren't the toughest opponents, so that shouldn't give them an unfair advantage. Actually, ant species can do a lot of crazy things like bite, sting, spray acid or explode themselves all of which I think I can do in DF (exploding themselves at a strategically optimal time might be impossible). I wouldn't consider an acid spray to be as out-of-character as thunderbolts or telekinesis, but I would want to limit its effect to just pain and nausea kind of things.
Not sure how I would do a corrupted magical artifact race. That sounds like a suggestion for Masterwork, it seems a little out of place in FuFo. Some kind of a cursed Leokin race like D&D's Tyrfing or WoW's Undead could be interesting if I were drawing them, but with words, Saber-tooth Leokin is a stronger description. I guess they don't have to stay lions though, a race of Demonoids or Devil-kin could replace them instead. I've never been a huge fan of Angel-vs-Demon themes though, so something like Ogre-kin or Djinn or Asura would be more interesting to me. Of these designs, I still like Saber-tooth Leokin the best. Maybe STLKs could be ageless or incredibly long lived like Yeti to suggest that they are remnants of a time before time. Some sort of corrupt/demonic physical trait could be fine, but I don't know what it would be. Maybe extravision.
Also, the Golden Age has dawned! I guess it just had to wait until the first day of a new year. In 6 years of this fortress, I haven't actually broken into the caverns yet. Maybe I'll try to slay some Forgotten Beasts while we're here.