Eh… well, the thing about Meganium is that her main problem is a bad movepool, and Mega Evolution doesn’t actually do anything to fix that. Adding a new type to her would be nice, but it’s also kind of difficult because she doesn’t really learn any attacks outside of Grass and Ground (due to the rule that Grass-types Don’t Get Nice Things), and I don’t know if Ground makes sense for her. Mostly Meganium trades on her support attacks – Aromatherapy, Leech Seed, Dragon Tail, that sort of thing – so buffing her defences is kind of a no-brainer, and a nice defensive ability like Filter can’t hurt. Something like Grass Pelt might be more flavour-appropriate, but it’s not like Filter really has a consistent link to any particular kind of power anyway (I mean, Mr. Mime and Mega Aggron? What’s the common theme there?).