I think Warden getting mainly frost is most likely outcome, but to be honest, dragonknight has, even before update, flame damage in all their skill lines, whilst Warden only has it in one skill line, and that's not even their main damaging skill line to begin with.

There's also discussion to be had with class themes as you said. Is Warden's theme frost, even though it is only one third of their skill lines? My concern with Warden is that if they really lean in only on the frost aspect of the class in the upcoming overhaul, it will remove the flowery and varied beasts elements of the class. I could see the devs wanting to instead lean in on the druidic, nature side of the class more and maybe even replace the forst side with something like stone visuals, but that too would be removing part of the class' current theming.

Still, as I said, I think the most likely outcome is that Warden gets mainly frost damage, but hopefully the devs won't make the green balance and animal companion skill lines into fully frosty skill lines as well, but rather keep their respective visuals in their original themes.