Well how about it, Just like holiday npc's? Dan could make some Seasonal Npc's, For Summer/Winter/Fall/Spring.

And they would stay for the whole season. It seems like a novel way to introduce more and varied content....

Seeing this inspired an idea.
The Mizers, a race of beings that are divided amongst 4 tribes. During the different seasons members of certain tribes will travel from their home worlds and attempt to raid and steal as much as they can before the next season begins. Each tribe specializes in different technology and produce different rewards, so if you disable the same type of Mizer ship during different seasons you'll get different rewards.