Apologies if I'm being dumb and this already exists.
I think a rain item, similar to
this blizzard item from Christmas 2015, would be nice.
It could probably be released for Easter or something, since spring is generally rainy. To make a full item set, there could also be stuff like puddles, rain clouds, rain boots, raincoats, and possibly umbrellas, though I'm not sure how most the linearts would hold an umbrella.