Good slate. Gonna do a big evaluation of how I think each one will affect the meta below 'cause I have a few free hours.
Fairy-/Ghost-type Cresselia:
Fairy/Ghost is very unique typing to say the least, and it does wonders for Cresselia. It trades its weakness to Bug- and Dark-type attacks in exchange for a 4x resistance to said type, immunities to Normal-, Fighting- and Dragon-type attacks and a weakness to Steel-type attacks while retaining its old Ghost-type weakness. All in all this is... actually a pretty cool trade-off. After consideration of Levitate, it has a whopping four immunities. Yes you heard me: FOUR! The defensive utility that this provides immediately apparent, with it being able to cockblock a huge range of Pokémon including (but not limited to) Garchomp (252 Atk Life Orb Garchomp Iron Tail vs. 252 HP / 252+ Def Cresselia: 182-216 (40.9 - 48.6%) -- what is damage?), Lando-T (+6 252+ Atk Landorus-T Stone Edge vs. 252 HP / 252+ Def Cresselia: 328-387 (73.8 - 87.1%) -- guaranteed 2HKO after Leftovers recovery, although tbf it does this already outside of SD+3 attacks w/ KOff, meanwhile: 0 SpA Cresselia Ice Beam vs. 56 HP / 0 SpD Landorus-T: 248-292 (74.4 - 87.6%) -- guaranteed 2HKO after Leftovers recovery), Weavile (252 Atk Life Orb Weavile Iron Tail vs. 252 HP / 252+ Def Cresselia: 172-203 (38.7 - 45.7%) -- guaranteed 3HKO after Leftovers recovery, and Moonblast just blows it back)... if it is physical and doesn't get STAB Steel- or Ghost-type, it gets walled. Simple as that. Meanwhile, special attackers get walled by its SpD set. Oh: and as if to rub salt into the wound this thing can spinblock too! All in all, this thing seems insane.
This thing really doesn't have many weaknesses tbh. The only real drawback that I can see with it is that its recovery option (Moonlight) can be easily stalled out by things which can deal close to 50% to it due to its low PP, and Excadrill does a number on it too considering that sand support is able to limit its recovery and considering that, while Exca can't OHKO it, it doesn't take much back and can SD in its face.
If this were to become part of the OU metagame, sand offense would almost certainly increase in usage. Rain also has a very good matchup versus it for its ability to limit its recovery and overwhelm it with powerful Hydro Pumps. Specs Politoed may make a return for its ability to both limit its recovery and break through it with Choice Specs, rain-boosted Hydro Pumps. Cresselia+Gastrodon will most likely be a good core, and Jellicent+Cresselia double-ghost seems like a very viable strategy that could revive full spikes-stack stall.
On top of all this spectacularness (is that even a word?), the typing makes a lot of sense thematically. Fairy-types moves are commonly associated with the moon, which is backed up by Cresselia's access to Moonblast and Moonlight as well as by its design (which is very obviously based on the moon).
VERDICT: Most likely borked.
Bulletproof Doublade:
This is a really interesting addition for one reason. It is able to counter non-Hex Gengar due to its immunity everything barring a random HP Fire or a resisted Icy Wind, and by running the former Gengar sacrifices its ability to speed tie with Lati@s--which is huge considering how good the synergy of Bulletproof Doublade with Lati@s is. If you've played Inheritance you know how much Bulletproof improves Doublade on its own, and while it loses out on Spikes here it also has access to STAB Iron Head and Swords Dance, allowing it to go either offensive or defensive.
In addition to this, it matches up well versus Alakazam due to its immunity to Shadow Ball, and similarly it now counters all variants of Reuniclus. Add this to its already good set of stuff that it checks/counters and it is a powerful addition to the metagame.
This fits well on stall, balance and BO, and as such if this were to be added it'd definitely encourage said playstyles to thrive. Cores like ChanBlade, DouBliss, LatiBlade, GengBlade double-ghost etc. all seem like powerful cores utilising it. Its main weakness is definitely the lack of reliable recovery I think something like Recover or even Moonlight would have done wonders for it. However, all in all it looks very solid.
VERDICT: Very good
Grass-/Fairy-type Flower Veil Shaymin:
This is very interesting in deed. With it providing it an immunity to status and stat drops as well as an immunity to Dragon-type attacks and STAB on its key dragon-targetting attack, it definitely sets itself apart from both Celebi and Serperior. With Seed Flare and SubSeed it is able to very easily cheese past a lot of its so-called stops, and it also likes having that ability to maim dragons without running a piss-weak coverage move, which is sweet considering that it resists Earthquake from Garchomp and outpaces stuff like Hydreigon.
Compared to the above two, however, it doesn't really feel like anything special. Like, while the T-wave, Toxic etc. immunity is nice and all, it is very rare for stat-dropping prevention to come in handy on a special attacker. In addition, it is still largely overshadowed by Serperior due to the fact that it directly reverses negative stat drops, which makes it a straight-up better answer to stuff like the rare Sticky Web and allowing it to directly benefit from Defog (lol what is evasion clause?) in addition to it being straight-up faster without a boost. All in all, I don't think it would have a particularly big impact on the meta either, as outside of checking dragons better and having usable physical bulk/not being bait for T-wave Thundy/Klefki it doesn't really offer much over Serp.
VERDICT: Nothing special
Unaware Mandibuzz:
This is... ok. Like, it still suffers from a lot of the same problems that Mandibuzz faces and just offers an alternative to Quagsire and Clefable really. In addition to the obvious typing differences, Clefable it has reliable recovery and is an okay stallbreaker with Taunt (don't use Defog--Mandibuzz is a crappy defogger because rocks weakness and because sacrifices an important move for Defog) and over Quagsire it has the aforementioned Taunt. Honestly, I'm not big on this. It would have been better in the Hoopa-U metagame, but honestly atm it feels... like just another Unaware 'mon.
It only feels worth using for role compression. While Quagsire has a useful Electric-type immunity, Mandibuzz doesn't check much by comparison. Quag covers a lot of what it checks already, namely countering non-Grass Knot/Taunt Bisharp. Compared to Clefable, who can still stallbreak if its desparate with Calm Mind (not that Unaware+CM is very good because it either has to run the god-awful Moonlight or sacrifice coverage/utility for WishTect while being easily worn-down by status due to lack of MG), and it also sacrifices WishTect and cleric support, which stall adores. In addition, in order to run Unaware it loses its ability to remove the need for sleep fodder versus Amoonguss and Breloom's Spore, which sucks ass.
There would probably be no meta shift with this, and it pairs with all of the same stuff that regular Mandibuzz does too. Not really anything else to say here.
VERDICT: Average at best