Really digging MegaGross. As others have pointed out, Tough Claws seems like the most likely candidate for a new ability. Which combines nicely with Clear Body for a switch in without fearing Intimidate and Sticky Web. Levitate seems plausible, but Beldum and Metang were already shown floating without it.
As far as stats, a large Attack increase seems inevitable. An increase in defenses also seem likely, and a small boost in speed would be possible, even a small boost like Tyranitar's +10 would be helpful.
Metagross's typing has its uses, but some added bulk would help it against the common weaknesses he has. His physical move pool is pretty diverse, Meteor Mash, Bullet Punch, Thunder Punch, Ice Punch, Earthquake, Pursuit etc. Steel has decent neutral coverage, and is definitely an under prepared for type, since its only really useful with STAB.