Okay. Where does it say that these work?
In regards to dragonborn breath weapons,, It was a question that Jeremy Crawford answered.
His response was basically that, if the creature is able to exhale, it can use the breath weapon.
Rage requires that the creature be able to rage. I'm not aware of any creatures which are incapable of doing so.
Smite requires being able to attack....
Fast movement requires being able to move, so that should work with pretty much anything other than a tree.
In contrast, an arakkoa's flight wouldn't work if the new form doesn't have wings.
Why I see the plasmoid as potentially different than the arakkoa is because the arakkoa loses a physical feature, while the plasmoid's ability is to generate a physical feature where one did not previously exist.
For animals, I can see the argument against allowing it to work because I think it's somewhat logical that an amorphous form or oozey quality be necessary.
Air, Fire, and Water elementals are similarly amorphous (in so much that they don't have a physical body in the same way a normal terrestrial creature would). I'm on the fence about the Earth Elemental. While it's more solid, I don't see elementals as being as beholden to specific humanoid form.