Thoughts on everything...
Thick Fat Togekiss: The first thing that comes to mind for me is that it becomes one of the best switch-ins to any Charizard, as both Megas fail to 2HKO with their Fire-STAB on a any defensive set. In addition, even though Thick Fat didn't change this, a physically defensive set also avoids the 2HKO from Jolly Banded Talonflame's Brave Bird, while still handling both Charizards. Kyurem-B still beats it with Fusion Bolt, and Heatran and Mamoswine will run Flash Cannon/Stone Edge more, respectively, to beat it. Not really much else that changes here outside of potentially handling Ice coverage better, but most Ice Beams I can think of right now can also run Thunderbolt.
Slack Off Rhyperior: Obviously reliable recovery is great on a defensive Pokemon. The comparison to Hippowdon is perfectly valid, since the only thing Hippo would have over this guy is some extra special bulk and a different typing. The choice to use them would probably come down to typing, with Rhyperior having extra Steel, Fighting, and Ground weaknesses, but also Normal, Fire, and Flying resistances. In this meta, I feel like the Flying resist is overall more helpful, allowing him to better handle Talonflame and Mega Pinsir, and potentially Mega Charizard X if they lack Outrage. As I might have hinted before, Hippowdon will probably go Specially defensive to help cover threats like Heatran better than Rhyperior would.
Dragon/Steel Druddigon: Interesting, the neutrality to Fairy and Dragon certainly seems beneficial, and he does get STAB Iron Head to abuse the new typing. Outside of the obvious support capabilities with all the new resistances, it could be used as a Fairy-killer for other Dragons. Togekiss and Clefable need heavy physical defensive investment to avoid an OHKO from Banded Iron Head, and Clefable can't even guarantee it. Sylveon and Mega Gardevoir are absolutely destroyed. Azumarill becomes even more useful of a Fairy due to its neutrality to Iron Head and access to Superpower to hit one of Druddigon's new weaknesses. Mega Mawile is another interesting one, as a Banded Earthquake will likely OHKO Mawile (but not guaranteed), while Mawile could still OHKO with Play Rough (though not guaranteed). What makes it interesting is that Mawile barely outspeeds Druddigon. Speaking of which, his low speed will be his biggest obstacle on Banded sets, while lack of reliable recovery somewhat limits his supporting opportunities.
Prankster Gourgeist: Well, this guy is pretty much like the perfect combination of all the other Prankster users we've seen so far. It takes SubSeed from Whimsicott, Will-o-wisp from Sableye, and Destiny Bond from Mega Banette, and though it likely won't use all of these at the same time, it does have more bulk than any of the other Prankster users (assuming Super sized of course). He also gets unreliable recovery in Pain Split and a decent 100 Attack for a wall, although it's stuck with weak moves like Seed Bomb, so Taunt from opposing Pranksters could be problematic, though it does outspeed Sableye, so he could get a Leech Seed/Wisp off before getting Taunted there. Special Thundurus seems like a decent answer, having a faster Taunt, not caring too much about Will-o-wisp and being able to 2HKO with HP Flying unless Gourgeist goes fully specially defensive, which is an option with priority Will-o-wisp I guess.