We have seasonal bedding just because it's flannel, not because of its pattern. Our flannel sheets are fish, a woodsy bears/moose/cabin pattern, and a cloud pattern. The rustic ones are hand-me-downs from my ILs who decorated their old cabin in a rustic theme.
LL Bean and Pottery Barn have pretty good flannel bedding. I saw some cute Target prints last year around Christmas, too. Just thinking about this makes me yearn for the day when I can turn off the a/c....