Thanks to PsychicNature for suggesting this Mega!

Mega meganium would gain a large boost to its defensive stats and a boost to its sp attack. Regenerator would be Mega meganiums new ability, allowing it to restore Hp upon switching out. Its looks would change to where leaves now grow all over its body, aiding in photosynthesis and portection. It would get thicker skin and a spiky ridge on its back.
Stat boosts:
40 defense
40 sp defense
20 sp attack