I already listed a few megas in this thread, but there's more I thought about:
Pokémon I think deserve a Mega to help them and/or they're also cool:
. Mega Pidgeot (the original bird is the worst of them... in battle of course...)
. Mega Nidoqueen & Nidoking - they're not that bad, but their stats could use a bigger buff than +10 in one stat; plus, they look perfet for Mega Evolution to me for some reason...;
. Mega Marowak - to continue the Kangaskhan-Cubone conspiracy... now, seriously, give it Huge Power so it doesn't miss its Thick Club signature item, and it's ready to go...
. Mega Jynx - I still think it should have a real evolution like Magmar and Electabuzz but it seems they gave up on those, so a Mega would suffice...
. Mega Lapras - Such a high BST, yet it sucks...
. Mega Snorlax - It's not what it used to be anymore... and it's a fan favorite anyway...
. Dunsparce - I agree with what was said before... give it an evolution and then a Mega evolution...
. Mega Steelix - It's really cool, and needs a high buff to its Attack stat...
. Mega Granbull ? - It's now a Fairy-type, and with some stat buffs, it could actually be good...
. Mega Magcargo - give Solid Rock, buff up that Sp. Atk by 60 points, and its defenses a bit, put it in Trick Room... profit...
. Mega Octillery - looks to me like a counterpart to Magcargo (even though, they're nothing alike, maybe it's because they're molluscs)... anyway, it's one of those pokémon with potential but fall a bit short of actually being dangerous...
. Mega Mantine - I'll explain this one later...
. Mega Kingdra - another one with high BST, but no awesome stat, plus, it would complete the group of pokémon with starters-type + dragon-type (after Charizard X, Ampharos and Sceptile)
. Mega Ninjask - not sure, I like it, and I'd love to see it surpassing Deoxys-S' speed...
. Mega Exploud - another one who needs a bit higher attacking stats to be usable...
. Mega Torkoal - don't know why, but there's people asking for it and I think it fits the theme..
. Mega Flygon - ultra cool but needs more power...
. Mega Milotic - as a counterpart to Mega Gyarados (this one seems one of the more likely megas to happen, I believe), and give it fairy-type...
. Mega Walrein - may not be evident, but it some sort of counterpart to Aggron and, like Lapras or other stage 2, high BST but doesn't excel in anything...
. Mega Luxray - more speed, more speed, much more speed... and it's cool...
. Mega Floatzel - give me a reason to keep my trained Floatzel from Platinum... cause it sucks the way it is (I'm even thinking about catching a new one in platinum, train it again and transfer to gen 6 and replace the one I have)...
. Mega Druddigon - it's one of those pokés who are not likely to ever have a regular evo, and it's kinda strong but not enough, so a Mega would be good for it...
. Mega Golurk -
http://fc00.deviantart.net/fs71/i/2013/231/e/b/mega_golurk_by_dragonith-d6iufsk.png
. Mega Bisharp - looks cool, has a good attack, interesting type combo, but its speed sucks and its main moves have low damage output... also fits the concept...
. Mega Braviary - more because of the coolness factor but it's not that threatning as it is now...
(No Megas from Gen 6 for now)
Pokémon I think should get a Mega... just because they're freakin' cool:
- Arcanine: give it Speed Boost
- Legendary birds: when XY started leaking, there was a rumor the birds could Mega Evolve... it turned out to be fake but since they decided to put so much Kanto in Kalos, including the birds, they should be the first legendary trio to get Megas...
- Dragonite, Salamence, Metagross, Hydeigon: pseudo-legendaries, 'nuff said...
- Crobat: It's one of my favorites... give it 8 wings...
- Skarmory: unnecessary buffs at their best... also the reason to give Mantine a mega too, they used to be counterparts...
- Wailord: make it be 200 meters long...
- Drapion: it's just cool...
- Weavile: it's popular and has the same problem as Bisharp when it comes to moves...
- Rhyperior: It already looks like a Mega... Imagine it with 4 drills! 2 smaller cannons under the arms! An even bigger wrecking ball at the end of the tail! So much Digimon, it's not even funny!
- Heatran: that legendary that doesn't really behave or is treated like one, so a Mega would not seem strange...
- Zoroark: Lucario
- Haxorus: 197 Attack stat! The other 50 points can go to Speed and Defenses, but speed is preferred. Haxorus is awesome, period.
- Volcarona: If it looked dumb, I'd say give it Simple! Instead, give it Drought... it's the Sun pokémon (and is Fire-type unlike Espeon)...