I really really love Chan's idea, I think this would add an interesting dynamic. As is a lot of people write in extreme weather into posts, but even something as simple as a little line at the end of seasonal descriptions that shows the status of natural disasters, diseases, etc, would be really cool. IE, a flood in a particularly rainy birdsong that blankets a lot of helovia in water, etc.
As for a shop, I'm inclined to agree with and back up Tamme--I've never seen it go well. That said, Snow, I know the Aurora Basin holds an achievement based contest every season that has a similar point based system for some small seasonal and crowd sourced rewards (small items, art, etc.) They are fun and have some of the qualities you are talking about but are herd specific, and I don't know if other herds do it but I'm sure they could if they don't already.
I agree that drops can be tricky and they require a lot of patience to get what you want, though, I'm not sure they couldn't be improved upon as they currently stand.
But yeah, I'm on Tamme's side anti-shop (At least site wide, I agree that a plot by plot basis would be cool,) I've always seen it as distracting and taking away from the writing and turning things into more like a systemized "game" rather than a collaborative creative experience.
Now... -returns to thinking of own idea-