Ah, very interesting topic, Scrafty! All of these Pokémon were once very good in NU, and analyzing them in a very different metagame offers very interesting discussion! All of these Pokémon (imo at least) have the potential to be threatening in the NU tier with proper support, or even without it!
Mesprit
Mesprit was a titan in NU before it left, and I think it would be very good even now. During its stint in NU, Mesprit functioned as a fantastic user of Calm Mind, as well as being an extremely effective weather supporter, with access to both Rain Dance and Sunny Day, as well as both U-turn and Healing Wish. To be honest, while Mesprit now faces plenty of competition from other Psychic-types, such as Gardevoir for support, Musharna as a Calm Mind sweeper, and Jynx as a Choice Scarfer, it still carves a niche for itself in being very bulky and versatile. One thing that lets Mesprit down is its base 80 Speed stat, which is just not fast enough to outspeed common threats as it was while it was still in NU. Other than that, I can see Mesprit getting plenty of use in the current meta, with a strong array of very balanced stats and just the right movepool.
Klinklang
If you think Klang was effective in Stage 9, just see how cool its older brother would be in this coming stage! While the gear was in NU, it was a fantastic sweeper, using its unique Steel-type STAB and amazing boosting move in Shift Gear to do damage to plenty of unprepared teams, with just enough coverage to be a hassle to stop. In the most recent stages of NU, Steel-types have been becoming rarer and rarer, meaning that less teams prepare for them (which sort of kind of lead to Klang's effectiveness, imo). Klinklang brings many useful resistances to the table, making it very easy to set up on the likes of Jynx (if you run Lum, of course), Samurott, and Kangaskhan. But, aside from how amazing Klinklang looks on paper in this meta, it struggles to beat out many of the bulkiest walls in the tier, such as Tangela and the new drop in Mandibuzz. It still has the same old coverage issues, being hard-walled by a lot of Ground-types, such as Golurk, who threaten it immediately with STAB Earhquake, and is immediately threatened by fast Fighting-types in Primeape and Sawk, who easily revenge kill it if it's not behind a sub. Aisde from that, Klinklang can be a force in the tier with the proper partners, such as a fast Water-type capable of blowing away bothersome Grounds such as Simipour or Floatzel.
Cryogonal
Introducing a halfway decent spinner to NU! No, I'm just playing, but in all seriousness, Cryogonal brings to the table the fastest Rapid Spin in the tier. To be honest, mono-Ice typing sucks dick, but Cryogonal's speed, power, and overall decent bulk allow it to spin and threaten a variety of spinblockers, such as Golurk and Haunter, with STAB Ice Beam. Not much else to say on the matter, though, as I never really used Cryogonal all that much :/.
Rhydon
Finally, we have a Pokémon with the potential to be the best Stealth Rock setter in the tier. Rhydon's fantastic mix of great Attack and good physical bulk, in conjunction with a set of cool abilities, makes it a very interesting setter of Stealth Rock. In conjuction with the obvious strength and weaknesses its Ground/Rock typing provides, Rhydon gets access to Megahorn, which allows it to dent Grass-types hard, something not many Stealth Rock setters can boast. Another interesting tidbit about Rhydon is its superior overall bulk and better typing to Piloswine, factoring in Eviolite of course. Imo, it'd be a really interesting addition to the tier.