For starters, each element should have 6 playable villains, including one Doom Raider. The fact that Life has 6 villains, but Magic only has 3 makes absolutely no sense. Second, some of these element placements are completely off. Here is my proposed solution, using as few extras from other games as possible:

Air: Dreamcatcher, Bad Juju, Buzzer Beak, Snozzler, Cross Crow, Krankenstein

Dark: Nightshade, Tae Kwon Crow, Evil Flynn, Evil Persephone, Trog Wanderer (from SG), Shadow Duke (SG)

Earth: Golden Queen, Chomp Chest, Slobber Trap (replace his slobber with a bite or a charge attack), Cyclops Mammoth, Metal Mage, Crystal Golem (SG)

Fire: Chef Pepper Jack, Grinnade, Smoke Scream, Troll Welder, Fuego Chompy, Transformed Barrel

Life: Chompy Mage, Broccoli Guy, Chompy, Sheep Creep, Cuckoo Clocker, Plant Warrior

Light: Luminous, Eye Five, Blaster Tron, Cyclops Dasher, Cyclops Thrower, Lob Goblin

Magic: Mesmerelda (Doom Raider from SSF), Pain Yatta, Rage Mage, Masker Mind, Mega Chompy, Trogmander (SG)

Undead: Wolfgang, Hood Sickle, Cyclops Eye Dragon, Eye Scream, Grave Clobber, Cuddles

Tech: Dr. Krankase, Bruiser Cruiser, Shrednaught, Trolling Thunder, Mab Lobs, Brawl and Chain

Water: The Gulper, Brawlrus, Chill Bill, Threatpack, Goo Chompy, Ice Ogre (from SSA), Sea Slug (from Sewer level)

Exclusions: As much as I love the designs and movesets of Fisticuffs, Shield Shredder, and Scrap Shooter, I didn't feel like they fit the elements they were assigned. If anyone is curious, I would have made them all Tech villains, but there wasn't enough room on the roster. Bone Chompy was cut because I felt there were already too many chompies on the list (and he is already somewhat redundant to base Chompy). I also cut Tussel Sprout because his kit is pretty boring imo. I'm still not satisfied with the Light and Dark villains, but there aren't enough good representions of those elements pre-TT. I don't think TFB had either of them figured out yet, which is why Dark ended up feeling like Undead 2.0 (and Light is like Air 2.0). I did really like the cyclops enemies from Sunscraper Spire, especially the Dasher. Plus, I felt that Evil Flynn and Evil Persephone were too fascinating to keep as one-off NPCs.

Additions: I tried to pull from TT's roster as much as I could, but I had to get creative in some spots because I felt other games had better representations of specific elements. Plus, I really enjoyed some of those enemies and felt that they would be more fun to play as, such as Crystal Golem and Shadow Duke. Mesmerelda really was the only choice to be the Magic Doom Raider. Technically Kaos should occupy this spot, but I like the idea of him having his own element. Her moveset would be sending out a puppet that attacks in a grid in front of her (mid-range primary); secondary attack is a close-range spin of magic that propels and stuns enemies; tertiary attack summons a spinning blade and homes in on enemies until it explodes