Alright so I keep looking at what I submitted before and it doesn't really do what I wanted it to. So here's my revised submission, for better or worse:
Rainmaker still builds up the clouds by pseudo-trashing. I did like the limit on the old one, but I don't like the sound of "Discard up to 3 Rain from Exile" which would be the equivalent here. Anyways it
gains a Rain in addition to the Exiled one, if you Exile like a Silver or a Duchy, which opens the floodgates and you can have a nice storm to bless your crops.
Notes/Edits: Rain costs $2 so that you can't just Exile it with Rainmaker to get extra Rain.
Rainmaker says "Exile a Rain" because I can't say "from the Supply" if it's not in the Supply, and "from the non-Supply" sounds stupid to me. -- took Ethan's "from its pile" suggestion, thank you.
Rain ended up pretty similar to Figurehead but that wasn't the goal. At first it was a Treasure that gave +2 Cards, but that was basically a slightly worse Horse. This had more of the feel I was going for: it stacks up for simultaneous effect, rather than playing and returning in a chain like Horses/Spoils.