So; inspired by Fervor Invoked - how about a keyword that is "This creature has the current evergreen ability of its colour"

The abilities for each colour can be spelled out - and can then change with the environment.

There are a lot of evergreen keywords in each color though, even if you restrict it to the primary ones.

Yeah; but I envisage there just being a list of which the five current signature ones are, which rotates over time. Probably as one of the advert cards in each set.

Heck, maybe other cards could interact with it. "Sorcery: Green's signature is deathtouch ueot"

This seems like a possible mechanic for a conspiracy style set: Archetypal Aven.

Mmm, that seems nice.