OK, finally got a chance to update these suggestions. I still haven't figured out the best wild card, though. We'll start with these.
Mega-Abomasnow
NEW STATS:
+10 Def, +10 SDef
Defensive boosts seem like they shouldn't matter much for the Abominable Snowman, but giving him mixed defenses of 90/115/115 makes him quite the mixed tank, giving him easy switch-ins to bulky waters and fairies without fearing any of their attacks. For example, assuming a defensive spreads of 248 HP/252 Att/8 SpD (just made up on the spot now - there are probably more ideal spreads), he can more easily switch into threats like...
- Swampert, whose strongest attack now deals 13.8-16.4%.
- Florges, whose Moonblast deals 26.1-30.8% despite her nice base 112 SpAt.
- Mega Ampharos, whose Dragon Pulse from 252+ SpAt only deals 41.7-49.6%.
- Mega Absol, who can no longer OHKO even with the 4x effective Fire Blast, and whose Sucker Punch deals only 33.6-39%.
- Suicune, who can't 2HKO with a +6 Scald (36.2-42.8%), and who is 2HKO'd be Seed Bomb in return.
- Celebi, whose Psychic deals 22.1-26.6%.
Basically, if you don't have a super-effective attack (and in some cases, even if you do), Abomasnow probably won't be 2HKO'd by you, and can almost certainly 2HKO you in return or set up a Swords Dance. Going up to 115/115 defensively isn't a huge bump, but honestly I think that Mega Abomasnow is pretty good already (and vastly underrated), so I'm hesitant to give this guy too much of a boost. (Note that in the OU version of theorymon, they gave him Technician, which turned out to be a pretty serious offensive boost - Technician Ice Shard, Bullet Seed and Frost Breath is no joke!)
Feraligatr
NEW MOVE:
Belly Drum
Look at that belly! Gator is just crying out for a Belly Drum. This will make Gator a far more threatening offensive powerhouse, especially with a +6 Aqua Jet. Its low Speed will keep it from being overwhelming, however, as it will be easily revenged by things like Hydreigon.
Assuming a Jolly BD set with something like Belly Drum/Waterfall/Aqua Jet/coverage move (EQ is nice for electrics and bulky waters, Ice Punch hits dragons and Hydreigon in particular, Crunch is your best option aginst folks like Celebi), you've got calcs like...
- +6 252 Atk Feraligatr Crunch vs. 252 HP / 0 Def Celebi: 600-706 (148.5 - 174.7%) -- guaranteed OHKO
- +6 252 Atk Feraligatr Earthquake vs. 236 HP / 0 Def Mega Blastoise: 321-378 (89.6 - 105.5%) -- 37.5% chance to OHKO
- +6 252 Atk Feraligatr Earthquake vs. 252 HP / 252+ Def Suicune: 245-289 (60.6 - 71.5%) -- guaranteed 2HKO after Leftovers recovery
- +6 252 Atk Feraligatr Aqua Jet vs. 248 HP / 0 Def Crobat: 271-319 (72.6 - 85.5%) -- guaranteed 2HKO after Black Sludge recovery
- +6 252 Atk Feraligatr Waterfall vs. 252 HP / 232 Def Florges: 462-544 (128.3 - 151.1%) -- guaranteed OHKO
- +6 252 Atk Feraligatr Aqua Jet vs. 0 HP / 0 Def Mienshao: 340-402 (125.4 - 148.3%) -- guaranteed OHKO
- +6 252 Atk Feraligatr Aqua Jet vs. 0 HP / 0 Def Honchkrow: 379-447 (111.1 - 131%) -- guaranteed OHKO
It would be a pretty solid threat against stall, who it can outspeed most of and tear through, and does decently versus offense as well thanks to its +6 Aqua Jet. Its decent defenses and typing give it more opportunities to set up than, say, Slurpuff, as well. On the other hand, it wouldn't be unstoppable, especially because even with Jolly it's relatively slow, and it will only have three available moveslots. Carrying EQ means that things like Celebi and Hydreigon can respond to it. Carrying Ice Punch or Crunch makes bulky waters like Empoleon and Mega-Blast able to threaten it out. It can also still be revenged by faster threats that resist Aqua Jet. In other words, it would play similar to the stallbreaking Swords Dance set, but would actually have use over Haxorus in this role thanks to more immediate power and access to Aqua Jet.
Metagross
NEW ABILITY:
Levitate
(Sorry, had to include a picture of a levitating Metagross.)
It gets Magnet Rise, so obviously this guy is capable of using his magnets to float. Metagross suffered hard from the Steel nerf, so giving him a new immunity will help salve his wounds a bit. This will give him more opportunities to switch in and allow him to check threats like the Nidos, making him an interesting offensive alternative to Bronzong.
Levitating Metagross with an Assault Vest could be a fantastic answer to many special sweepers and the Nidos in particular, being immune to both of their STABs and able to threaten them away with Zen Headbutt (a guaranteed OHKO against Nidoking, and a 62% chance to OHKO Queenie). Its Stealth Rock set would also love a free switch-in on ground attacks. Heck, the CB set would love that too.
Giving it Levitate wouldn't completely outclass Bronzong, who would still be more useful in some niches for its superior defenses, but it would give Metagross the boost he needs to be useful in his myriad roles once more.
Meloetta
NEW TYPING:
Fairy/Psychic
Trading her Ghost immunity for a Dragon immunity and neutrality to Dark will make Meloetta a fantastic answer to non-Iron Tail Hydreigon. Despite its lower power, Dazzling Gleam gives a handier second STAB than Hyper Voice. Though she isn't pink, she fits in with the fairies thanks to her pixie-ish appearance and dancing theme, and the fact that she already has access to Dazzling Gleam makes this an easy conversion. I mean, heck, she looks like she could be sisters with Gardevoir, the other Fairy/Psychic.
As for Meloetta versus Gardevoir, I honestly think non-Mega Gard belongs in UU anyhow, so having a Fairy-typed Meloetta would help replace the hole she's left. Gardevoir has a better movepool anyhow (in particular thanks to having access to Moonblast instead of just Dazzling Gleam as her Fairy STAB), and a better ability in Trace. Meloetta in return is bulkier and faster, but her Serene Grace is largely wasted when her best moves are things like Psyshock and Dazzling Gleam, and her slightly better SpAt doesn't make up for the lack of Moonblast (which deals ~6% more damage from base 125 SpAt than Dazzling Gleam does from base 128).