Sand Force Mega Tyranitar: We've had this suggested multiple times, but it never really saw the light of day for a reason I'll not speak of. However, it seems that the planets aligned just right this time around and allowed this Theorymon to get slated. I wasn't really liking the idea at first for several reasons. First, Mega Tyranitar would only have up to 4 turns to sweep with a Sand Force boost after setting up a Dragon Dance, and that's only either when using it as a lead or after a teammate kicks the bucket. After the sand wears off, Mega Tyranitar is left with no real ability if not running a Hippowdon; at least after sand wears off, Mega Tyranitar could whip up more sand upon switching if it still had Sand Stream. Which leads me to my next point, that being the fact that the sand can provide Mega Tyranitar with extra special bulk to take a hit, whether it's switching into an attack or taking an attack as it sets up. But as time passed, these cons seemed less and less significant to me, and the real pros were starting to shine, like being able to use Rock Polish to outrun opposing Dragon Dancers, or having an absurdly powerful STAB attack to muscle past walls if using Dragon Dance. Just take a look at this:
+1 252 Atk Sand Force Mega Tyranitar Stone Edge vs. 252 HP / 252+ Def Cresselia in Sand: 240-283 (54 - 63.7%) -- guaranteed 2HKO after sandstorm damage and Leftovers recovery
Yeah, so unless your wall resists Rock, you likely have nothing to safely switch into this monster. Having a powered up Earthquake is also neat for sweeping, as most Pokemon that could have resisted Stone Edge will take severe damage from Earthquake if your predicting skills are good enough. And while the sand Tyranitar itself summons doesn't last long, the potential carnage it creates in those few turns can be devastating if left unchecked. Overall, while I was certainly skeptical at first, I've actually warmed up to this idea, and wouldn't mind it if this won.
Water Absorb Forretress: This one might seem a bit odd at first, but I think there's some merit to this Theorymon. Ever needed a safe switch-in to Scald? Forretress is now able to easily jump in on Scald, a trait that very few Pokemon without Natural Cure or Heal Bell/Aromatherapy can boast to have, and even heals 25% of its health when taking Water-type moves. With Water Absorb, Forretress also finds some good switch-in opportunities against a lot of Pokemon, like Keldeo, Azumarill, Manaphy, Starmie, Gyarados, Kabutops, Kingdra, Suicune, and Tentacruel, and can then safely Volt Switch to either punish a switch or to bring in the appropriate answer to the threat at hand. For example, Thundurus can take out Manaphy fairly easily but doesn't enjoy getting hit with Ice Beam, so switch Forretress in and take the Ice Beam with no problem (no hax, plz), and simply use Volt Switch. It's a win-win scenario here, since if the Manaphy switches out, you can bring in an appropriate check/counter for the new threat, but them not switching means you can safely switch Thundurus in and KO the Manaphy after the Volt Switch damage. Plus, Forretress is plenty slow enough to abuse Volt Switch to provide safe switches except when against Slowbro, so this strategy is great when facing Water-types, especially rain teams. This also eats away the time that rain is up, so Forretress is a great time waster against rain teams. Not a great Theorymon on paper, but I definitely think that this is worth the team slot if you hate Water.
Technician Mega Sceptile: I'm really liking this Theorymon. Like, I REALLY like this one. There aren't that many new special attacks to take advantage of, but Dragon Breath is a nice new STAB that could be annoying with its paralysis, and having Hidden Power acting as either Flamethrower or Earth Power is a luxury on its own. However, the real variety comes from its physical movepool, with great new STAB attacks in Dual Chop, acting as an Outrage that doesn't lock or confuse and can break through Sturdy/Focus Sash and can still deal damage to a Substitute, and Bullet Seed, which breaks what Dual Chop does, but can potentially have more than 50% more power. Improved coverage moves include Rock Tomb, Aerial Ace, Bulldoze, and a more consistently powerful Low Kick, so combine this with Swords Dance, and a physically offensive Mega Sceptile doesn't sound too absurd. Losing out on Volt Switch and Prankster Thunder Wave immunities does stink, but I feel that the new power that Mega Sceptile receives greatly outweighs this. A simple buff with a few new positives, but that's part of why I like this one.
Grass / Rock Tangrowth: I'm a bit skeptical of adding a Rock typing with the intention to buff a Pokemon defensively, but I think Tangrowth is the right candidate for the new typing. With a new Rock typing, Tangrowth loses its Water, Grass, and Ground resistances and gains weaknesses to Steel- and Fighting-type moves, so things like Keldeo, Landorus, Mega Metagross, Azumarill, Mega Scizor, and Excadrill now have an easier time against Tangrowth, so I was a bit unconvinced at first sight. But when I realized that Tangrowth could act as a neat defensive pivot of sorts for sand teams, I started to like it. Gaining all of those weaknesses is unfortunate, but shedding its weaknesses to Fire-, Flying-, and Poison-type moves is very nice, and combined with the extra Special Defense gained by the sand, the Assault Vest set is a pretty reliable answer to things like Latios, Talonflame, Mega Manectric, Raikou, and Dragalge, and having a special wall in general is a useful thing for sand teams. But as I've said, the drawbacks that the new Rock typing bring are a bit rough, as the new weaknesses get in the way of it being a good switch-in to most things with Focus Blast. Other than that, I kinda like this idea.