I'll like to see Mega Linoone gain a large boost in attack, and speed, with minor boosts in defenses. As for it's ability I see it getting either Vital Spirit or Normalize as it's ability. Also perhaps become Normal/Electric
Mega Mightyena I'll like to see get a boost in speed and defense. I can see it getting the ability Huge Power or Strong Jaw, if Strong Jaw then it should get a boost in attack too.
Mega Pelipper should get a boost in special attack, and special defense, and a smaller boost in speed. Also the ability Gale wings or Rain Dish will match it best.
Mega Swellow I'll like to see get a large boost in attack, a slight boost in it's defenses and speed. I can see it having Gale wings too.
Mega Gallade (as Ralts is an early, but rare mon) get a large boost in speed and attack (30 each), and slight boosts in each of it's defenses (20 each). I can see it getting the ability Super Luck or Technician.
Linoone doesn't even look like an Electric-type to me, and Normalize sucks as far as abilities go. It would be like, completely useless against Ghost-types.
Love your Mega Mightyena suggestion, Huge Power could be a nice ability for it. And I think Pelipper really does need a boost, so nice to see you make a speculation for a Mega Pelipper as well.
A Mega Swellow with Gale Wings... may not be as potent as Guts but we all know how great Talonflame was thanks to those increased-priority Flying moves. And I know how much people want Mega Gallade (especially given Gardevoir already has a Mega that exists), and I think the two abilities you mentioned look nice although I don't think it learns much that takes advantage of Technician.
But I know everyone at this stage wants a Mega Flygon... but there's still the chance the Get TV broadcast could pull the rug out from under us. Remember that Slowpoke, Audino and Lopunny all got Megas and we weren't expecting them?