So there's Fresh Pumpkin, Freshly Picked Pumpkin, Pumpkin on the Vine, and Uncarved Pumpkin. Carving an "uncarved pumpkin" will only give out items from this year. It seems like fresh pumpkin, on the other hand, will give pumpkins from a few different years (including the brand new ones).

So how does it work? Will any pumpkins except this year's give carved pumpkins from all previous Morostides? If so, if I horde a bunch of uncarved pumpkins and wait, will they at some point start generating all possible pumpkins too?

no, I've tested this. Only the Fresh Pumpkins will give out various year pumpkins. Every other type of carvable pumpkin only gives out its specific year. There may be a few older ones I haven't tested because I don't have them.

Cool. (Weird, wonder why they do that :P) Thanks so much!