I really don't think they should add hydration/thirst, like Ben said, it'd be micro managing hell.
Personally I believe hot weather should be harder than cold weather, but less resource-expensive. In cold weather you need heat, and that requires fuel, heavy clothing and food as your hunger increases, while in hot weather you have to fight off heat. Don't stay under the sun, don't do hard labor: I know from experience heat can sap all your stamina out of you, so a change to walking speed instead of hunger rate could make sense.
In cold winters things don't grow, but you can work on things like smithing no problem, in hot summers the debuff to plants and animals could be reduced (if not eliminated), while physical work raises your heat, meaning you can do less of it. Trading the need to stock up on resources with a slower development.