I know that seasons are at least vaguely defined in the future of H&H, so this is not a suggestion of "add seasons" and more of a discussion of what effects seasons may have and how they are implemented. My thoughts:
Even though a hDay takes only 8 rHours (that is, a haven-&-hearth-Day = 8 real-world-Hours), this still works out to four rMonths for each full cycle of seasons in the game. I think this may end up being a little slow. Ideally, one would be able to play a single character for several hYears, so the lifespan of a normal character will put a limit on the length of the hYear. I would suggest one hYear = 4 rWeeks as a rough estimate- spending a week in each season.
Beyond speeding the day up more, one could instead make the year consist of fewer days or months. If we kept the same length hDays as we currently have and had week-long seasons, each hSeason (3 hMonths) would consist of 21 hDays, and each hMonth would be 7 hDays long.
We can, of course, assume that the Hearth-Solar-System is convienient enough to keep the Moon in step with these seasons.
The main obstacle to seasons, I think, is going to be weather, which is going to be slowed because pretty much all large items will need to have different graphics for when it has a little or a lot of snow on it, for when it is baked in heat and for when it's flooded with rainwater, and so on. In fact, it might be that tying weather to seasons directly and having the weather effects be the only direct influences on gameplay may be the way to go.
For example, instead of having Arctic Foxes spawn only during winter, have them spawn only in snow tiles, which form whenever it snows, which only happens during winter. This way, if it's a mild winter (the RNG deigns to have less snow than average), then there will be fewer Arctic Fox spawns than in other years.
Other weather effects:
Seasonal Crops (No wheat in snow) : Flooding (Might be tough, probably work like snow tiles): Snowed-in roads (If there are snow tiles, easy to make them slow carts and PCs): Icebergs (besides tiles, the other enemy of boats) : Wildfires (Summer hates you) : Animal Behavior (Good news; it's bear mating season!) : Long-term crops (Plant crops in spring, harvest in fall) : Temperatures (Too cold, need more clothes, die/Too hot, need more stamina and beer, die)