if we're talking about abilities, I want to mention Solar Power. It gives a 50% boost to SAtk in sunlight, at the cost of 1/8 HP per turn.
If we did something like that, we could use it as a special attacker pretty well, though it would suicide from all the damage after a while. However, with that big boost in SAtk, you wouldn't need a massive SAtk basestat to let it clean things up, at least after you get rid of special walls. That way, you could also have a big attack stat, letting you use it as a physical attacker outside of Sunny Day. That would make it pretty unpredictable, which is cool, but also limits it a lot so that you can counter it.
To make sure it isn't broken, we'd have to sacrifice some stats somewhere. Speed would have to be high for it to be usable, but you can nerf one of its defenses, and make it bulky on the other side.
I dunno, I'm kinda thinking as I type, what do ya'll think of that?