Just had this idea in my head and maybe it could work. Seasons come and go like eras but faster. On winter there are more snow tiles and the tundra tiles become snow tiles. But during this season snow tiles are not completely useless. Maybe there could be projects (or done automatic) in cities where you set up local trade routes that make the snow tiles productive or something. And on winter the ski resorts get a nice boost. On Summer the opposite happens. Less snow and tundra tiles can get a little buff. The seaside resorts get the nice buff and food/animal tiles can get a buff too. With time of the year being a thing it would be interesting to have days your people celebrate to give you certain yields. Fall/Spring can be when Summer and Winter slowly transitions to each other.