How about seasons or atleast snowy maps. Think about blizzards and ice tiles.
Snow might add alot of work on the art side though.
By the way, Loving the invention looking constructions. reminds me of the torture device in Princes Bride.
Yes we'd love to have seasons. As you say it would required lot of new art, but I think we'll do it anyway, just don't know when. Also we'd had to redefine when the plants grow and make fruit, and it would make a lot more sense.
Animal reproduction would also occur at a certain time of the year, as well as some disasters (for example lightning storms are more frequent in summer).
Winter would be a hard period with less food to gather and firewood to consume to stay warm.