Create a unique weather condition which requires Sunny Day to be used after Rain Dance. It causes Grass-type moves to gain an increasing for three turns. If Rain Dance is not recast, their power decreases every turn. In addition, Leech Seed's effect is doubled and it gains another ability which prevents the seeded Pokemon from switching out.
This is meant to mimic the growth conditions of Plants as well as their ability to wither and dry up. It could make for some unique Fire/Water/Grass teams as well as some unique Pokemon that take advantage of it (like an overgrown hedge that increases its Defense and Special Defense every turn that this condition is in effect).
while I do like the upgrade for grass types but the condition seems too complicated. And also, it would nerf sunny day teams because then they won't have there weather out when they try to overwrite a rain dance. If both weather is out at the same time, like I said, it sorta over complicates the game and makes keeping track of the weather and what each weather does more complicated for newer or younger kids.
Perhaps a weather condition isn't the best implementation. I was considering it as a separate ability for specific Pokemon, but I wouldn't want it to override another possibly more useful ability. I was really liking the idea with nurturing the Leech Seed to become more powerful, but I agree, it is quite impractical.
I do think Grass should get a unique buff from Rain Dance. One thing I can think of is having the ability Growth raise Sp. Atk and Sp. Def sharply while in Rain. I think that is a bit more fair and practical than my first idea.