-Swampert/Gastrodon should be able to handle Keldeo, Gastrodon perhaps a bit more due to Storm Drain and Recover. But since rain won't be as prevalent, Swampert could fare well against Keldeo too now.Thing is that Avalugg and D-nite are SR weak and along with Swampert, the core gets destroyed by Terrakion, Keldeo, SF Landorus, Thundurus(-T), among others I'm assuming. Mono-ice is just so hard to build around.
-I don't reckon Landorus will get past Avalugg even with SF. The numbers have already shown that Avalugg can take some SERIOUS beatings on the physical side, even from SE attacks. Landorus has no STAB to threaten Avalugg with, hits physically, and is OHKO'd by Avalanche.
-Terrakion is trickier, but numbers posted above showed that Terrakion would only 3HKO Avalugg with Close Combat, while Avalugg would OHKO with EQ 50% of the time. Stone Edge got nerfed thanks to critical hit nerf this gen and is weaker and less accurate than CC without the critical, so Avalugg shouldn't have to worry about that either.
-With the Hidden Power nerf, I'm not so convinced that Thundurus-T will be as problematic for Dragonite as it would've been before. I'll run some numbers, but I think ExtremeSpeed Dragonite will be able to deal with the thunder genie.
Thundurus-T is a pain for any core, though. You'll get nowhere against it by being defensive - you just have to outspeed and outnuke it, usually. I'd have to think about this one, but at least I still have 3 other Pokemon to choose from. I suppose SapSipper Goodra could replace Dragonite, I don't think it would have much trouble against Thundurus-T. It's not as threatening as Dragonite (or is it?), but for a core, it was one of my other top choices to go with Avalugg and Swampert/Gastrodon.
But yeah, Ice is hard to build around. Possible, though, and hell of a lot of fun. Even if Avalugg is OU material, as others have said, he won't make it there due to people freaking out about building around an ice-type. That's fine, those people will be easy prey for the iceberg.