A Marloss line for the triffids probably won't be happening, if you consider how different fungus and triffids are.
You have to consider, the Fungus has to use local wildlife (iincluding survivors) in order to expand itself over a wide area. We are basically it's carrier. It can certainly do something similar on it's own by making fungoids and using spires, but it's a lot slower than just infecting a few animals and letting them run wild. The Marloss Man is basically the pinnacle of that - you have shown yourself to be 'worthy' enough, you will be made into a living puppet of the fungus, a being so thoroughly infected by the fungus that it effectively became the fungus.
The triffids on the other hand do not use the wildlife for expanding at all. They seem to connect to the local Flora instead of Fauna, turning brushes, trees and similar things into their tools and little helpers (...why do I have an Ultima Deja Vu now? Oh, right, THE GRASS is attacking). They do not interact with the Fauna in a meaningful way which would suggest you could become one of their puppets.
To put it into another way: The Fungus is basically like the european explorers, finding america or exploring and exploiting africa. They are more than willing to take over the land by force, and to enslave the local populace (aka us/the fauna) in order to further their goals.
The Triffids on the other hand probably see in us little more than local wildlife. We are basically like a poisonous snake, a wild elephant, a lion or something similar. Yes, we are a potential danger to their 'people' and colonies, but we're just animals. We have to be taken down in order for the colonies to thrive.
If there ever will be a way for us to interact with Triffids, it would have to be by mutations. Have enough plant and plant-like mutations so that the triffids would recognize you as Flora and potential ally instead of stupid animal. That way they might have an interest in you. Maybe they'd try to domesticate you. (who's a good boy? Plant-boy is a good boy, yes he is! *pat pat*)
That would beg the question though, why not making other 'questlines' for other mutation trees too? We do have slime-like mutations, right? If you'd go far enough, could you actually interact with the zombies or the goo? What about animal-mutations for interacting with wildlife? What would stop you from doing a rat-king and gather all the animals of your corresponding mutation-line around you?
Hell, if you think about it, some combinations of mutations could be enough for beings like the Thing, Amigara Horrors or cthulhu nightmares (aka flying polyp, mi-go, shoggoth ...) to interact with you.