Stats: Increased Attack, maybe more HP Ability: Antithetic (Between the user and the foe, all attacks with super effective and not very effective types are inversed) Typing: no change New Moves: no new moves
Commentary: Antithetic might be the answer to normal typing's entry to the top of the food chain, with weaknesses and strengths inversed, normal attacks become super effective against rock and steel pokemon, while defensively, fighting becomes not very effective against the user. Immunities are kept the same way. With this, a properly setup Mega Linoone can become a red code level threat for the opponent.
Stats: Increased Attack, Increased speed to tie with base 103 Ability: no change Typing: Water / Dragon New Moves: no new moves
Commentary: Gyarados finally becoming dragon is a thing I expect if GF decided to give him a Mega evolution. Gyarados is a pretty strong OU sweeper as it is, there is no urgent compensation it must acquire "by all means" so I didn't give him many buffs. Still, let's not forget you can only have one Mega in your party, and as such, your trump card shall have a mild buff in attack and speed.
Stats: Increased HP, Special Defense, Attack Ability: Battle Hardened (takes 125% damage from super effective attacks, and 150% from 4x super effective attacks) Typing: no change New Learnable Moves: Nasty Plot, Power Gem, Stone Edge
Commentary: Although it has the looks to get a dragon typing, I think he should stay with the same typing, that's what makes him the Aggron we all know and love. With increased defense and the removal of 4x super effective hits, he no longer has the need to test waters in fear of being assassinated in one turn, while it can still be hit, he can finally take his role as your tanker, taking hits and giving some in return. Aggron also has the Nidoking Syndrome, it's an obvious physical attacker, but its Sp. Attack is not horrendously low and its special movepool is larger than your average physical sweeper, so why don't we take that to its advantage and maybe even boost it in the process? Nasty Plot to surprise the opponent; Ancient Rock would in most cases satisfy most people with the probability to increase all the stats of a Mega pokemon by one stage, but some might root for that extra power from Power Gem; and without Rock Head protecting him from the recoil Head Smash gives, we can see an alternative in Stone Edge.