Personally, I'd like Mega Golem to get back the arms he lost evolving from Graveler.
This is exactly what I mean. Back when the first two generations of Pokemon were around, Golem looked awesome and powerful. And then came Gen three with Aggron and Deoxys and suddenly Golem didn't seem so cool looking and became rather bland to look at. I mean we now have OTHER Rock and Ground types that look cooler and better than old Golem. I mean we have Rhyperior (who can shoot out rocks and occasionally a Geodude from its hands), Archeops, Tyrantum, Rampardos, and Bastillion who have great designs. And Golem? It basically looks like a lizard stuck in a mirror disco ball covered in mud. Yeah. It definitely needs a Mega Evolution.