Though based on its canonical nature, it would probably have something to do with merging with a triffid heart.
Post tresh Plant is the closest you can get to being a triffid. You will even get dreams of “a good looking triffid offering you a bouquet of human heads” so its pretty obvious that you are much closer to the triffids than to humans.
Otherwise a triffid version of the Mycus mutations is highly unlikely, if only because the triffids have no interest in humans at all.
I agree with @Junknown but maybe a mutation thats goes further down the plant path? Maybe turning you into a real life dryad?
Maybe with the ability to heal by joining with a tree, just as an example. Or does the line go this far already?
I was thinking of something that allows you to manipulate plants, whether you become one or not. You could walk through roots unimpeded and command a triffid queen to spawn a bunch of trees between you and a horde of zombies. I guess that would be overpowered though.
We can’t heal by joining trees and I doubt an ability like that would make it in the core game but that sounds like it would be an awesome mod ability for elf-a mutants. It gives you a way to counter disintegration while enforcing the whole “we should be a part of nature” theme elf-a has going by making them have good regeneration in the woods but rapidly declining health should they leave their forest home.
The obvious has already been stated: the Plant mutations already do something a lot like that.
I’m new to the game, so it may be that I’ve missed something, but, if we did want a way to get closer to the Triffids, the one thing that I would suggest adding is a Plant mutation that makes Triffids perceive you as one of their own, call it “Plant Pheromones” or something.
If that wasn’t enough and you really wanted to do one further, maybe add another mutation to make it possible for you to take control of Triffid creatures to some degree (after all, NPCs aren’t the only companions you can obtain; there are pets, livestock, robots {aren’t there?}, zombie slaves, and friendly blobs).