@ Water / Ground Hippowdon
This thing would be even more metagame defining as it is now, it could even be the number 1 used Pokemon in OU. With this typing Hippowdon would be able to better handle defensive Politoed, Heatran, Weavile, Gyarados, Volcarona, Cloyster, Kyurem-B, Kyurem, Jolteon, Thundurus-T, Rotom-W, Keldeo, Latios, Latias, Magnezone, Mamoswine, Starmie, and Tentacruel. Hippowdon would undoubtedly become the best weather inducer, with its ability to check or wall ~half the metagame plus reliable recovery. Being able to switch into defensive Politoed without worries (as long as you have a cleric in your team) is a godsend for sand teams, taking some pressure of from the bulky Grass-type that they usually carry.
And as for HP Grass on Politoed and Solarbeam on Ninetales it's really not that much of a deal. Defensive Politoed's Scald has 240 BP against Hippowdon while HP Grass has 280, and HP Grass on defensive Politoed would be very hard to fit anyway. On more offensive sets Hydro Pump and even Surf do the same or more damage against Hippowdon, so HP Grass is really a non-issue, or at least nothing that Hippo didn't have troubles with before. As for Ninetales, it needs to use Sunny Day as Hippo comes in, telegraphing the use of Solarbeam and making it very easy for Hippo to avoid it.