I think a Sableye Mega would likely see a big increase in defenses, much opposite of what Mawile saw with a big increase in attacks.I'd love a mega Sableye too. It was pretty crushing to see mawile get one and Sableye not, but GF could (and probably will) give him one this gen. There isn't a whole lot Sableye needs fixed though. I mean to be honest it's pretty powerful/annoying now, but maybe some extra power in its stats are in order. I do think it should keep it's typing and abbility if it were to get one.
(I really only want a Mega Sableye because of nostalgia.)
Mawile's Base Stats v. Mega Mawile
Sableye's Base Stats v. Mega Sableye
And I wouldn't mind if M-Sableye kept its ability as Prankster.