All of these are pretty good ideas. Here's what I think:
Fur Coat Mamoswine: For offensive sets, Mamoswine would actually rather like this buff. Taking on the threats alexwolf mentioned and more is pretty important, and with it, neutral uninvested Defense hits 392, which is amazing for an offensive Pokemon. But I feel lead sets would still prefer Oblivious, since it protects Mamoswine from Taunt when laying down Stealth Rocks. I can see Fur Coat sets with Leftovers or Assault Vest, so he does certainly have some use, but I'm a bit sceptical at the moment.
Tough Claws Drapion: I can see some justification in his use, with powered up Pursuit, Knock Off, Crunch, Aqua Tail, Brick Break, and Poison Jab, as well as the elemental fangs, he could certainly dish out the damage, and since most attacks he uses involves direct contact, we can usually just assume his attack is 362~ if you run Jolly with max Attack EVs, and 397~ running Adamant. Getting Swords Dance is pretty neat, too. But his only decent speed might force him to take a tanking role, so I'm not sure if he'll be the monster some people make him out to be.
Poison Heal Jellicent: Now this is a concept I can get behind. Poison Heal on literally any defensive Pokemon susceptible to Toxic is a godsend, but Jellicent utilizes this rather well. His sizable defenses and access to Recover are pretty amazing, and having resistances to Fire, Water, Ice, Poison, Bug, and Steel, as well as having Normal and Fighting immunities, Jellicent has many opportunities to activate his Toxic Orb. But being a defensive Ghost-type is a very risky thing this gen, with the popularity Ghost- and Dark-type moves have gotten these days, and being weak to Knock Off and Pursuit of all things is bad news. Overall, a great buff, but might yield so-so results.
Psychic / Normal Reuniclus: Another concept I could easily get behind. Psychic-types have always been a shaky choice when it comes to defensive types, being weak to Ghost, Dark, and Bug, but the Normal-typing will rid Reuniclus of one of those weaknesses, breathing new life into the lovable blob. This will allow him to literally tank some of OU's top threats, including Gengar and Aegislash. Losing a Fighting resistance stinks a bit, but Ghost immunity while not being weak to Fighting? Yes, yes, and more yes. But still being weak to Dark is a big problem, and Pursuit and Knock Off are his biggest nemeses. And without Defense investment, Trick Room being set up, or a perfectly timed Focus Blast, Pokemon like Bisharp, (Mega) Tyranitar, Crawdaunt, and (Mega) Absol can make your life hell. Despite all this, though, Psychic / Normal Reuniclus could be a monster in the right hands.
All in all, I think my vote will have to go to Psychic / Normal Reuniclus, since we need Pokemon to take on Ghost-types these days.