First time i post in this kind of discussion thread, but i like a lot the concept of this one, that's pretty interesting to discuss about the things that can or not come in future and compare the actual metagame with another.
Recover Claydol:
Firstly, Claydol is a Pokémon i've played a lot in the past, but regarding to the tiers changes the metagame doesn't let him be as cool as before, especially due to it's typing. Even with Recover, Claydol would be pressured too easily by water/grass/ice type who are a lot more played than before. I also would like to point that access to Recover doesn't let him the opportunity to run a coverage move such as Shadow Ball (unless you are running another rock setter but then i think others spinner could do this job better) even if it beat common rocker, it's pressured by other rockers like Torterra or Carracosta if it run Psychic, or common defog such as Skuntank if it run Psychic, Mantine, Pelipper or Shiftry.
Rapid Spin Carracosta:
Carracosta is actually a pretty cool rocker, because there isn't a lot of rockers that can use Scald, bar Carracosta, Omastar is the only one, and we all know how this move can be helpful in game, in addition Carracosta's decent bulk and Solid Rock make it a lot better than Omastar as a bulky rock setter in my opinion(purely mine, not trying to say thats better of course) but like Claydol, it's pretty easily pressured by common defog such as HP Grass Skuntank or Shiftry. I would like to say that Carracosta, as Adaire pointed, can spin against things like Tauros, Archeops, Swellow(non HP grass variant) pretty easily that is a big point.
Ice Beam Rotom-Frost:
Rotom-Frost is a pretty underrated Pokémon in my opinion, that can actually take advantage of opposing's hail from Abomasnow, but yeah Blizzard's accucary kinda sucks 70% is too low, but i feel like, the addition of Ice Beam wouldn't change these offensive set but giving a boost at it's defensive potential, even if ice type is pretty bad defensively, but the access to wow/pain split+stab bolt beam isn't that bad as it let you keep, even if you are defensively based, keep a better offensive presence than if you only had to rely to Blizzard as a Ice STAB, after, yes, I agree that difference of power change a lot, but the ability to hit all your move can help sometimes
Dragon tail Sliggod:
I've keep this one for the end because that's a thing i've always wish, Sliggoo is an amazing Pokémon that can easily fit in balanced/more defensively centred build. But as it's been said, Outrage as only STAB kinda suck for a defensive mon, as you need to be asleep or to wear down 3/4 Pokémon before being able to lock yourself into Outrage. While Sliggoo is pressured by ice type, Dragon Tail help him a lot to prevent them to come in due to the moves damages+the potential entry hazards (because i mean, if Sliggoo gets Dragon Tail, most of build with Sliggoo are going to run Spikes imo) with Dragon Tail, the only things that can be really annoying for Sliggoo would be Knock Off. In addition, as Adaire said again, Dragon Tail would be an amazing option that can stop set up sweeper that can be annoying such as Scyther or Work up Tauros.