Here's a defensive core that crossed my mind, it's more of a prototype at the momentso I'll definitely appreciate input and testing. I also don't know how to post sprites, if a mod could edit them in / someone could tell me how to do it, that'd be great.
Charizard @ Charizardite X
Ability: Blaze/Tough Claws
248 HP / 144 SDef / 120 Spd
Careful Nature
-Earthquake / Fire Punch
-Dragon Claw
-Will-o-Wisp
-Roost
Standard bulky WoW-ZardX, which has been discovered as a pretty good stall mon recently. Has the benefit of being pretty unpredictable pre-mega and even after, as ZardX can run multiple very dangerous sets.
Azumarill @ Assault Vest
Ability: Huge Power
16 HP / 252 Atk / 240 SDef
Adamant Nature
-Aqua Jet
-Play Rough
-Knock Off/Superpower
-Waterfall
Assault Vest Azumarill is pretty underrated imo, it got very good special bulk and only a few weaknesses, which allows it to switch into quite a few special attacks relatively unscathed. Make no mistake though, its attacks still put out a LOT of damage. Is mainly used as a switch-in to Lando-I who pretty much wrecks the rest of this core, even though it can't repeatedly switch into its attacks and should be careful of Sludge Wave on a predicted switch. Also has no form of recovery, so it might appreciate Wish support to be able to come in more often.
Ferrothorn @ Leftovers/Rocky Helmet
Ability: Iron Barbs
252 HP / 216 Def / 40 SpD
Relaxed / Impish Nature
-Leech Seed
-Protect / Thunder Wave
-Power Whip / Gyro Ball
-Stealth Rock / Gyro Ball / Spikes
Pretty standard defensive Ferrothorn, I guess by now we all know what it does. My personal favorite is the set which is slashed first, since it's really annoying to face and Leech Seed + Protect + Leftovers is actually pretty decent recovery, even though it doesn't mind Wish support either. Spikes can be chosen if your team has another SR setter already, Gyro Ball as a secondary STAB for more offensive presence might also be nice.
So, how does this core work? When building cores, two very popular types are Fire/Water/Grass and Fairy/Steel/Dragon. Guess what? This core is both. All three mons are more than capable to take a hit, 1.5 of them have recovery and they all can dish out some hefty damage as well. Ferrothorns Weaknesses are covered by Azumarill (and it's Fire weakness by Zard X as well), even more by its AV since most attacks aimed at Ferrothorn will probably be special anyways and Azumarill can threaten most Fire/Fighting users out. Its ability to switch moves comes in really handy as well as you might be able to get off a surprise attack on the next opposing mon, given that your opponent doesn't run calcs and/or mistakes you for the BD set. Azumarill's weaknesses are covered by Ferrothorn (1 immunity, 2 resists) and also decently by Zard X (2 resists). Zard X on the other hand has its Dragon weakness covered by Azumarill (immunity) and Ferrothorn (resist) and its Rock weakness covered by Ferrothorn. The only problem are Ground attacks which hit all 3 members of the core at least for neutral damage and are very common, but can also be worked around with the right teammates.
Between its very solid resistances/immunities and offensive presence, this core also provides hazard support, status support (as well as Leech Seed if you count that), Knock Off support and priority and as such is pretty well-rounded. Depending on the teammates chosen, the core can work in full stall or more balanced teams.
So yeah, that's my idea. As said, input and testing as well as threats would be appreciated, I didn't have the time to test it extensively so far (I don't have the time for extensive testing in general, which kinda sucks), but I think there's some potential here if I didn't miss something very important.