I've been thinking about how a system for the impact of temperature on hearthlings could be implemented to add to the survival challenge. Hearthlings would have a new bar indicating how well their homeothermy is going, with neutral in the middle ideal and hitting either extreme resulting in unconsciousness or even death, if they can't return to a safe hearthfire or recover by just not doing anything. New wound conditions like frostbite and sunburn might accumulate when this gets to the extremes of HHP damage, with SHP first to represent more mild conditions like heat exhaustion or numbness.
Each hearthling would have some ability to self-regulate to the centre of the scale at a certain rate when their environment nudges them toward one end or another, by either by expending their stamina to cool down, or their energy reserve to warm up. If a hearthling's ability to regulate their own temperature isn't enough, or they're too low on energy or stamina to keep up with the rate of change, the way to survive would be to change their environment or use tools to reduce their exposure, for example by building up a fire, jumping into water, or staying out of the wind or sun.
In the environment of H&H, not usually marked by sand dunes, the sun's rays aren't likely to ever be dangerous on their own, but at high noon in high summer, heavy exertion (and, especially, inappropriate clothing) could still make it add up to too much. This would make it a better time to be out on the water, or in a shady forest, instead of working hard out in the dust fields. Still, farmers with the right stats, equipment and supplies should be able to tough it out, especially if those supplies include frozen treats made using ice brought down from the mountains!
Warriors would find it to their advantage to wear less armour (and more wet warpaint!) in such conditions, and might find the amount of water they need to carry enough that it could guide the times and places they conduct their campaigns, encouraging night raids or tunnelling in - with underground being a good place to be during a winter siege as well. Those expecting to fight opponents with much better armour than they have access to would especially have reason to engage (or defend) where the heat is the worst, while in the extreme cold, it may be that only those wearing heavy furs (and equipped with skis or snowshoes, of course) will be good at fighting, even if they're not as well-armed.
At night in the spring and autumn, and for most of the day in the winter, being indoors, underground, or near a fire would be necessary to avoid spending extra energy on staying warm. Good clothing could mitigate that, while being wet could make it much worse. 'Soaking' could be a value with a cap that increases with each item of clothing worn (depending on their capacity for being soaked), and that can be quickly reduced by changing into dry clothing, or more gradually as part of warming up by a fire. Bad weather conditions would add to this value even for those who don't go into the water.
In hot conditions, the extra cooling effect from being soaked could be a benefit, or even a necessity for travelling through an inferno. Perhaps with a special garment that's designed specifically to have a high capacity for retaining water - along with such good insulation it'd be uncomfortable anywhere else - useful for entering burning buildings, or a hypohetical magma dungeon. To discourage warriors from too liberally making use of the cooling benefit of getting soaked just to reduce how exhausting it is to fight, it might be good balance if equipment that gets soaked tends to degrade unless it receives special care. Waxing or other waterproofing methods could protect from this, but also resist getting soaked in the first place... and such waterproofing might be essential for a basic cloak to be any use in a sleet storm.
A hearthling's stats could indicate whether they're better at staying warm in the cold, cool in the heat, or some of both. This in itself could be a reason to work toward characters with balanced stats so they can adapt to any conditions, and be an added wrinkle for min/maxed characters, who may find they can't function as well at the extremes and need more pampering.
Since new characters are the ones the most at the mercy of the elements, and the system shouldn't be irrelevant to anyone, higher stats definitely shouldn't just equal more temperature tolerance overall. Some characters might have an easy time staying cool, and therefore be more comfortable with brief periods of heavy exertion, but struggle to stay warm and therefore need a lot more shelter and clothing available, making them bad at roughing it out in the wilderness. Others would have a relatively easy time staying warm without shelter, but have to wear less in summer and spread their exertions out over a longer period of time. This could add a new dimension to the interactions between hearthlings with 'civilized' lifestyles and those without.
In addition to the extreme heat from molten rock or a fire burning out of control, there may be some mystical sources of extreme cold more severe than anything within common experience. Those who assume that going underground or indoors is always a good way to find shelter could, on occasion, find such places haunted by creatures whose mere presence makes a midwinter night feel like a summer day by comparison... a torch in hand could be the least of comfort in such a situation!