Water Shuriken Cloyster: This essentially fills in for wherever Razor Shell would be, and is a good attack on Cloyster in general. This also lets him get past... umm... Focus Sashers...? Even though Icicle Spear and Rock Blast do that too. Yeah, there are only two reasons that I'd ever use this over Razor Shell; priority and taking out Sash Mamoswine, and even then, I'd still prefer Razor Shell since just the threat of a 50% chance defense drop can seriously put pressure on my opponent. Not a bad buff, by any means, just not one that really sticks out for me.
Defog Whimsicott: Now this I can get into. Defog + Prankster is definitely an interesting combo to try out, and I think Whimsicott can excel at utilizing this. Defog is great at removing hazards, but another cool trait is that she also has Taunt, so she can get rid of hazards, and prevent them from being put down in the first place, which can put some pressure on defensive Pokemon. This isn't a dramatically great buff, but a good buff, nonetheless.
Electric/Dark Luxray: Flavorwise, this is excellent. Battlewise, almost equally excellent. Electric/Dark coverage is great, only resisted by a few Pokemon in the game, notable ones being Breloom, Hydreigon, Krookodile, and Chesnaught, though all four are hit super effectively by his coverage in either Superpower or the elemental fangs. Having resistances to Ghost- and Dark-type moves is highly valued this gen, and getting STAB on his Crunch helps a bit. Though his flaws in low speed and only passable bulk really put some pressure on him, and now he also carries weaknesses to Fighting-, Bug-, and Fairy-type moves. Though to fix his bulk problem, an Assault Vest/Intimidate combo set, coupled with a Guts boost, could really carry him the extra mile. I may have been biased about him in the past, but I'm gonna give him the benefit of the doubt on this one.
Scald Blissey: This sounds interesting, punishing physical attackers as they switch in and potentially crippling them. This would potentially make Blissey an unbreakable wall, capable of crippling any physically attacking Pokemon in the game aside from Fire-types and those with abilities that make them immune to Water (And our last Theorymon was Dry Skin Escavalier. Coincidence?). That said, Mega Charizard X, Conkeldurr, and those with Substitute, such as Kyurem-B and (Mega) Gyarados will still be problematic, and people may forgo Natural Cure for Serene Grace, just so Scald will burn 60% of the time, which really isn't worth it, imo, since the ability to get rid of status without using a move is amazing for longevity, and is an invaluable asset in general. And even if Scald burnt 100% of the time, 4-moveslot syndrome strikes again, with other options such as Aromatherapy, Wish, Softboiled, Seismic Toss, and Protect being very important, it can be a little hard to fit it in without losing an important move. A good idea, just a little hard to add in with just 4 moveslots.
My vote goes to Electric/Dark Luxray, since I feel that his presence would be the most interesting one to experience.