Victreebel
Type: Grass/Poison
Ability: Chlorophyll/
Gluttony → Arena Trap
Stats: 80/105/65/100/70/70 → 80/120/85/140/90/75
Victreebel is based on a pitcher plant, a sort of plant that lures bugs onto it, who then slip into its cavernous body and are dissolved by an acid in it. As such, I decided to give Victreebel the Ability Arena Trap (disclaimer: the Japanese name of Arena Trap translates "antlion." However, Diglett and Dugtrio certainly aren't antlions, and since the "pitcher" part of "pitcher plants" works similarly to an antlion's trap, I think this is okay). This, along with the buffs to Attack and Special Attack, allows Victreebel to function as a dangerous revenge killer. While it may be unable to boost its mediocre Speed with a Choice Scarf, it can use Sucker Punch to pick off quicker attackers. It also gets Giga Drain to recover HP when necessary. 80/85/90 defenses are decent, roughly equivalent to Eelektross, meaning it won't die to every little attack. I don't feel this is overpowered since Arena Trap fails to snare Pokémon who are Flying, Ghost, or have Levitate (and there are a lot of those). Additionally, it has a lot to fear from Flying and Ice priority, as well as any attacker who can outspeed it and kill it (or simply survive Sucker Punch).
Kabutops
Type: Rock/Water
Ability: Swift Swim/Battle Armor/
Weak Armor → Moxie
Stats: 60/115/105/65/70/80 → 60/150/120/80/90/105
Kabutops is a monster in the rain, but it suffers outside of it from mediocre speed and common weaknesses. As Mega Kabutops, it would function somewhat similarly to Mega Pinsir, having tremendous Attack and Defense and decent Speed. Moxie gives it a bit of an edge, though; as soon as it KOs something, it becomes an even greater threat. It also doesn't have to worry about Talonflame spoiling its fun, either. Normal Kabutops would still have a niche, as the doubled Speed in Rain is an invaluable asset. Potentially one could Mega Evolve once rain has been set up, allowing Mega Kabutops to use the doubled Speed for one more turn.
Drifblim
Type: Ghost/Flying
Ability: Aftermath/Unburden/
Flare Boost → Flash Fire
Stats: 150/80/44/90/54/80 → 150/95/64/105/104/80
Moves: Flame Charge, Overheat, Heat Wave
Maybe I'm the only person who's thought this, but I've found it strange that Drifblim, being based on a hot air balloon and all, doesn't get any Fire attacks outside of Will-o-wisp (and that hardly counts). While Mega Drifblim is still Ghost/Flying, it gains Flash Fire, the first non-Fire Pokémon to get it (also making it the only Pokémon with four immunities). It's not much of a game changer, but with buffed Special Defense and its new Fire immunity, Mega Drifblim gains a lot more opportunities to switch in. Drifblim has always been known as a good user of Baton Pass, and its Mega Form helps it there. It can use Flame Charge to boost its Speed before passing that boost (as well as massive Substitutes, Stockpile boosts, or Calm Minds) onto one of its teammates. Its slightly increased Attack and Special Attack also allow it to hit harder when it needs to.
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If anyone has any suggestions for these, I'm up to hear them. I had a lot of different Ability ideas for Mega Kabutops, including Tough Claws, Hyper Cutter, Sheer Force, Swift Swim, Sniper, and Adaptability. If it got Sniper I'd give it Razor Shell and possibly Cross Chop. I think Adaptability would be too strong, even though it fits right in with its Pokédex entries. Mega Drifblim was also a disappointment. Originally I had it with 115 Speed and Flare Boost, but even with max Special Attack (base 105) it was really weak, defeating its purpose. I'd have to increase its Special Attack more if I wanted to make Flare Boost work. I also thought about Infiltrator; it fits Drifblim's flavor really well and would give it something to do with its massive support movepool.