My opinions:
Fur Coat Mega Ampharos - Flavorwise: if scraggly Furfrou can get a defense boost from its unkempt hair, it does Mega Amph's totally stylin' hair no justice. Also, I also made this nomination to boost Mega Amph's viability without making it totally unstoppable, since it is still really slow, has no recovery, and is weak to common physical moves in Earthquake, Play Rough, and Outrage; even the slower attackers in Azumarill and Mega Mawile can totally take advantage of Mega Amph. A perk to Fur Coat is that it also affects physical-damaging special moves like Psyshock and Secret Sword, so Mega Amph can instead either focus on its Special Defense, reinforce its Defense even further, or making use of its newfound bulk with the likes of Agility.
Regenerator Registeel - Flavorwise: Eh, not really seeing it, but whatev. A Steel-type with that kind of passive recovery is great, but I'm just not so sure if Registeel itself offers as much as other Steel-types. Its support movepool is okay but Ferrothorn already seems to encompass the most of it. I can certainly see the appeal of a Steel-type with good recovery, but in Registeel's case, I find myself asking: "Okay, I get to keep this thing around longer....now what?" If it could at least recover on the spot, it could seemingly pull off a nice bulky boosting set but alas. The most likely niche I'm seeing out of Registeel is a Stealth Rocker that easily outlasts almost any Defogger, but that doesn't really resolve its issue of being a sitting duck most of the time, especially when common Ground-types are Registeel's best switch-ins (Excadrill anyone?)
Slack Off Krookodile - Flavorwise: If Hippowdon can Slack Off, I can kind of see Krookodile sunbathing in the same manner, except one of them is based of a vicious, bloodthirsty carnivore :/ . Now Krookodile, with help from Intimidate, can totally take on neutral physical attackers one-on-one, akin to Hippowdon's role, except Krook has more power over bulk.
Water / Fairy Milotic - Flavorwise: Yeah I can see it. Milotic makes use of this typing defensively, now able to take on stuff like Keldeo and XZard which would've stung Milotic a lot harder (+1 XZard has a chance to 2HKO 252 HP / 0 Def with SR in play though). Unfortunately it doesn't have any Fairy moves to make use of Competitive, which is a shame since Miotic would like better coverage.
I still like my Mega Amph idea, but (voting for oneself seems like bad taste) Slack Off Krook seems like it can do its main job much better with just that one move in its repertoire.