I wrote this on the train. I have some ideas for Gastrodon but I couldn't write any since I didn't know its base stats.
Mega Lopunny
Normal -> Normal
Cute Charm / Klutz / Limber -> Butterfinger Aura (Auto Magic Room upon Switch in/Mega Evolve)
65/76/84/54/96/105 -> 65/126/104/54/116/115
New Moves: Play Rough, Swords Dance
Lopunny has the all around fun (but not too good) combination of Klutz + Switcheroo to cripple opponents. Now, since Mega Lopunny can't carry an item, Mega Lopunny instead gains the ability to magically alter reality to make people's items as useful as its items are.
The stats are simple. My friend convinced me around the time Gen IV started that Lopunny was a rather strong physical attacker. Of course that was a boldface lie and my friend didn't know anything really. So, this Mega is trying to make it into an actual attacker that can use its vast coverage moves
Swords Dance is because it needs some way to boost its attack. Play Rough, well, while some might say its JUST Fairy coverage, many people, including me, think Lopunny is like a Playboy Bunny and I'm certain they play rough if you know what I mean...
TROLL ALERT! TROLL ALERT! TROLL ALERT!
Mega Drifblim
Ghost/Flying -> Ghost/Flying
Aftermath / Unburden / Flare Boost -> Slow Start
150/80/44/90/54/80 -> 150/100/84/130/94/40
Moves: Trick Room, Skill Swap as level up move, Wish
Hot Air Balloons aren't very fast. That is a fact. Sure they may pick up speed when the wind blows but that's going to take a while. And even then, there are WAY faster things out there than them. Mega Drifblim takes that to heart by being one of the only Pokemon with a negative ability to actually USE it to its COMPLETE advantage (Sure Entrainment Truant Durant is a thing, but it still has to suffer from it). It's simple when you look at it. A super slow Pokemon...with Slow Start...and Trick Room and Skill Swap. Hmm...what shall I do with that?
Effectively, go cripple the opponent's team with this. It doesn't matter how, but this thing has got the defenses to survive a lot of stuff and do what you want to do. It's got Wish for healing. And it can get rid of its negative ability with Skill Swap or use it for even more fun with Trick Room. Use Skill Swap and Will-o-Wisp on a physical attacker to completely shut it down if you want! That's a fun thing...