Okay, I'll chime into the discussion.
What if Shadow Tag Lampent was released?
This would be really cool actually. Lampent could run a choice set to revenge a number of threats, such as Amoonguss, Roselia, Escavalier, Cinccino (without LO Rock Blast, of course), and a bunch of others. It could make a viable teammate to Pokemon such as Kabutops, Feraligatr, and Cinccino itself as well, trapping and killing many threats for teams. It also makes an excellent check to hail teams, of which are a powerful force in the RU metagame. Most teams that dislike hail mons such as Glaceon can really appreciate Lanturn's presence to remove them from play, and thus less worries about Blizzards at their Pokemon. I also see a potential SubCM set, with Flamethrower and Shadow Ball, as Lampent can come in on what it can trap while setting up CM in their face as they are completely helpless. This also works well with Memento Whimsicott / Uxie, as they can slice the opponent's offensive power in half, and the former can also set up Dual Screens as well, and with a helpless opponent, Lampent can come in and grab as many CM boosts as it wants. This wouldn't be overpowered, I don't think, but Lampent could do really well and be a viable RU Pokemon.
What if Torterra learned Shell Smash?
This would be really cool, the good old Rock Polish Torterra, except this would be that RP set on steroids. Torterra can become a very fast threat, and his EQ, Wood Hammer, and Stone Edge would be more powerful than ever. The great coverage and power between these moves will make Torterra a force to be reckoned with, and Pokemon that can and will sweep you at a moment's notice. It has a real load of power, and it has enough speed to outpace a lot of the metagame and have a lot of power. If Moltres is not too concerning to you, you can also run MixTerra with HP Fire or HP Ice as great options to run, to handle Tangrowth quite handily, although again this leaves it vulnerable to Moltres. It would be great on FWG cores, with partners such as Emboar and Carracosta both being viable. Bulky Smash Torterra might also be a possibility, as HnC said. Overall, I think this would be a legitimate threat in RU, and should see some use in the tier.
What if Sigilyph learned Nasty Plot?
This would bring Sigilyph's sweeping potential to another level. This means Sigilyph can run a full on NP set that can threaten many teams, while it can become a threat more quickly. As said, Tinted Lens may also be a possibility, as it would have some pretty much unresisted coverage, but Magic Guard+LO is also good for Sigilyph to hit hard with no damage costs. I think this would be pretty good.
What if Abomasnow was RU?
Hail teams are already great as they are, but Abomasnow would make hail teams better than ever. Abomasnow would summon hail to make Walrein, Glaceon, and Rotom-F suddenly real threats while having a wear down timer for everything. However, unlike Snover, Abomasnow isn't dead weight outside of support, for Abomasnow's quality of having actual stats means that Abomasnow can pose a legitimate threat too. He has great 92 offensive stats, two awesome STABs, and a good offensive movepool, so he can put some real opening dents for Glaceon, Walrein, and Rotom-F to thrive better. Abomasnow has great STABs that will send the two best Pokemon in the tier, Slowking and Druddigon, crying for mercy. I can see a Mixed Ebelt set with Blizzard, Wood Hammer, HP Fire/EQ, and Ice Shard, that can cover most of the tier pretty well. Blizzard and Wood Hammer together cover most of RU, and Druddigon and Slowking respectively will lose to Abomasnow one-on-one, while HP Fire handles Escavalier. However, EQ does work out of Entei if you're looking into that (the handling of Escavalier is useful if you're running Walrein as your hail abuser). I also see merit in a SubSeed set, as it can SubSeed+Leech Seed and hail stall, to be a bitch to take down, and stall defensive Pokemon such as Slowking. CB Abomasnow could also be a great physical wallbreaker to remove special walls for Glaceon, Walrein, and Rotom-F. This, along with creating a powerful playstyle while posing a real threat, makes me believe Abomasnow could easily be one of the best Pokemon in the tier.
What if Ferroseed learned Rapid Spin?
Ehh, I don't think this will do too much. While this makes a lot of sense given Ferroseed's in-game animation shows it rapidly spinning, Ferroseed will really need help if it wants to remove hazards. It can't do much to many of RU's viable spinblockers, such as Spiritomb, Rotom, and Golurk, due to its low attacking power, and its only hope is to use Leech Seed. It will need Pursuit support from Absol and Escavalier if it plans to pull off successful Rapid Spin attempts. Its lack of recovery and dependence on Eviolite mean that it will also get worn down by Spikes and SR. That said, I could see it working with Pursuit support, as its resistances and good bulk mean it can grab that opportunity to pull off a spin. You never know, really.
Anyways, gotta keep this discussion going on, so...
What if Mesprit got Nasty Plot?
Mesprit is already awesome but this would make it even more awesome, given it could make it a more powerful threat much more quickly than CM.