How about Thick Fat as an ability? Gives it a fire resist and makes it only 2x weak to ice, meaning with just a bit of bulk, it can take Sharpedo's Ice Shard.
This also allows us to move away from this pokemon being purely Sunny Day, but instead brings the threat of sunny day to the table. If you look at how Chlorophyll is distributed, you see there's already enough hard hitters to abuse with low speed. Now we need to introduce something that isn't 100% useless the moment Sunny Day is gone.
So for the two abilities I suggest Solar Power as the primary one, since it's currently neverused and undistributed period(unlike Chlorophyll). It also gives Sunny Day a new niche over Rain Dance: they aren't hindeed when the weather changes. By switching between Solar Power and Thick Fat, you can design your sweeper to be a complete power-house with Sunny Day(literally obliderating everything), or make it a good offensive one that fits on most teams by giving it Thick Fat. Extra resistance/ less ice weak doesn't hurt.