Hmm ... JF, that mike actually make an interesting challenge, I agree. Depending on the climate, the winter map in particular might be very different, since snow tends to wipe out a lot of features - it might actually require a re-draw of the base map. Late autumn would require quite a few bare trees, while the summer and spring versions could use similar bases ... I'm visualising a work flow with a base terrain layer for rock and dirt, a layer for grass areas, a layer for tree-trunks and bushes (sans leaves), a foliage layer, and a snow layer ... if there was water, then there should be at least a water and ice layer, and possibly a summer water layer for reduced water-flow if the region is dry in summers ...

If it's a "for sale" battle-map, there should probably also be variant terrain effects - snow in winter, mud in autumn (or spring) ... I'd be interested to see what comes up! Heck, it might be a fun challenge for me too!