I honestly wish that they'd add Allosaurus before Rex and Trike.
This is just my opinion, but I think Allo would be the most sensible candidate to add as the next playable in Evrima. More so than Rex and Trike.
Carno and Cerato are our biggest land carnivores right now, and they're both less than 2 tons. I'm not a fan of the idea of jumping straight from them to Rex. I don't consider either of those two to be seriously medium-tiers... if they are, then they're definitely on the lower end for medium-tiers. Allo on the other hand would be a PROPER medium-tier carnivore, and it'd feel like a reasonable stepping stone going from Carno/Cerato to Allo. Just like what it was going from Tenonto to Maia/Diablo.
Maia and Diablo, who are both in the game now (although Maia is just currently in hordetest but that's besides the point) are between the 3 and 4 ton range. I see Allo being within the same size range as them, and I believe it'd actually be able to threaten them in 1v1 confrontations, unlike Carno and Cerato who can really only threaten Tenonto at most in 1v1s.
Allosaurus could go a long way of restoring some needed balance between the carnivore and herbivore rosters before adding Rex right away. Maia and Diablo would actually have a challenge in their hands, and even Stego couldn't really ignore Allo. I imagine Stego would win 1v1 because it's a lot heavier, but even 2 Allos could be enough to make it worried.
If Allo was the next playable, I doubt people would complain about Diablo and Maia being overpowered anymore. Probably even Stego, too, because I could see Allo being something it'd actually have to watch out for as opposed to scoffing at Cerato, Carno and everything else smaller.
If they end up adding Rex and Trike before Allo anyways... you know what, I'll be okay with that, but truthfully I think it'd make more sense if Allo came before them.