I haven't seen anyone talk about this. But how about a winter season add-on that gives us more time to strategize, new monsters, perhaps new season abilities, maybe some special rules only in effect for the winter season.
What would a good winter season need?
Would it add unnecessary extra time to the game?
I think you've identified a possible design space that could be utilized by an expansion.
Thematically (and historically) armies didn't wage war (or even travel) during japan's short (but brutal) winters. In the existing design, winter is used as the moment when the warlords can lick their wounds and consolidate their gains (and determine who "won" the previous year's war). But there might be room for a 4th season to be the stage for court/shadow war, the final political manuevering to snatch victory from defeat, and honor from disgrace.
But I wouldn't expect (or even want) that expansion right now. It'll be best if it's built off the actual player response to Riaing Sun, so it can help correct minor issues and imbalances, and help build on the elements people like best.