My thoughts on some the Pokemon that are eligible for a Mega Evolution from Generation 3:
Mightyena-I would like it to get one. I don't think it has the best chance to get one but better odds than some others. I would imagine it would basically be an improved version of it, let it keep Moxie, give it a main boost in Speed and Attack, and whatever is left over gets split between the defenses. Something along the lines of Attack getting 30, Defense 10, Special Defense 20, and Speed 40, both Attack and Speed would be in top 100, and with Moxie it has a good way to boost its Attack even more.
Linoone-I don't really care if it were to get one. I don't really think it will get one. It's kind of in the same situation as Mightyena though(Both have the same bast stat total too). Attack and Speed getting the bigger boosts, and giving the rest to the defenses, although Speed could probably manage getting just 20, would make it in the top 30 in that case. Ability could use a change for something for convenient, such as Speed Boost.
Beautifly-Don't really care, but I think it may have a good shot. Again, boost in Speed and Special Attack, giving the defenses whatever is left. Ability change to something more useful.
Dustox-I would imagine if one were to get it the other would too, then again Dustox didn't get a +10 boost like Beautifly did so...who knows. I would imagine it would get main boosts in the defenses. Compound Eyes is kind of useless for it since most of its moves already have high Accuracy. Shield Dust could be fine on it but it could probably get a better defensive Ability, or maybe just get Prankster.
Ludicolo-I wouldn't mind it getting one. I think it has a good enough chance to get one. I could see Ludicolo just being improved on or being made to be used in a different way. I have a tough time seeing where exactly they would take it, keeping Swift Swim seems like the best option where it can ignore getting any boost there and let it go to other stats.
Shiftry-Again, if one gets it I think the other would too. And again, could see it improved or be used in a different way. It also has a Speed doubling Ability that it could just keep and let the Mega boost go to other stats.
Swellow-Wouldn't mind one, I could see it getting one too. It's Speed is already high, could let the boost go for its other stats, maybe it can lose a little in Speed and let it get more for its defenses, it seems better for it to just hit fast and hit hard though. Guts would be harder to use though, would be better off used in Double/Triple where you can use your other Pokemon to Burn it or whatever, so perhaps a different Ability, maybe Gale Wings.
Pelipper-Wouldn't mind one but don't really think it will happen. Seems better of being defensive, so improvement for it, increasing it's defenses and Special Attack, maybe even lose Speed to let it go to something more rewarding. Rain Dish should be fine on it for a defensive approach so it has more options to recover.
Masquerain-Would like one if it got back the Water typing other wise wouldn't really care. Don't really expect it to get one, and if it were to get one I don't think it would get the Water typing back. It has Quiver Dance so getting Mega boost in those stats would probably be the best. It would probably be better off with a different Ability, maybe Speed Boost.
Breloom-Don't really care but could see it getting one. I could see it getting an improvement or just get used differently. Technician stays or gets Iron Fist. Perhaps loss in Speed to let its defenses get more of a boost, and give Attack a little boost itself. Something along the lines of Attack getting 40, both defenses getting 40 each, and Speed drops 20, has Mach Punch benefit from either Ability and it's lack of Speed doesn't matter.