Mantises are an absolute joke. Among birds, you could choose Eagles, but I prefer Pegasus (I know it's not a bird, Morning!!!) or a bear (I know this animal doesn't have wings!!!) for poison. Ents are still not the best champion option. Maybe they'll make some kind of forest spirit?
P.S.: MEGA Ent is a different story :D:)!

Of course, it's a joke, but how many units are there for the role of an elven champion? Here, everyone suggests Ents, forest spirits. I suggest Sabre-toothed tigers (it's a joke, I know...). They will look like huge white tigers, only with shaggy fur and saber-toothed fangs. Or Ent champions (I wouldn't mind that in the Knowledge Base). I don't think Pegasuses will be champions, and it's not even a fact that they will be in the game at all. Bears can be added, but only as core units. Why not the best option? Agree, they will look more majestic than Pegasuses, and forest spirits are just Dryads. But the Daughters of the Forest are a different story...

Added 6 minutes ago

Nothing is known about the elves yet, but this is the most likely option. And we hope that it will be them :).

Phoenixes are neutral, and they don't really fit with elves :(.

Well, this is the most likely candidate for the role of the 4th race (and there was already a hint about elves). I support it; we are all waiting for the announcement.
Blue Phoenixes don't fit, but if you played parts 1-2, and even 4, then Phoenixes lived very well with fairies, elves, and unicorns (although they were ordinary, red ones).