On a personal note, if it would give Meganium a niche that could finally garner some respect from players, I'd be all for it, though I am apprehensive about the potential 4x Poison weakness.
If Fairy weakness is confirmed, I wish they would just take off Grass weakness to Poison then. Plants are not particularly more susceptible to toxins than we are, or bugs, or other animals. Instead, they are capable of filtering a variety of poisons and polluting components. For me, the argument of herbicides just doesn't stick.
And in terms of game, if Poison SE a type that counters Dragon, it would be more widespread and we should have all 5 threats to Grass all around. I think it's too much, as there are very strong types with 1 ou 2 weakness only.