More or less temperature will have more interactions in ALL aspects of the game including within combat, not ALL of these will be added, keep that in mind.
What this means for combat is that magics, weapons, fighting styles, and spirit weapons will have their share of effects on the temperature of the player ( could include but not be limited to variants of magics as well )
-for example a player that has been inflicted with soaked will have their temperature lowered or raised depending on the time of day (lowered at night on any island, +deserts are colder at night), and raised when hit with soaked in desert environment/ other environments especially in summer ( not including polar deserts with more research required ).
This player is then hit with wind magic ( for variant sake the cold variant ), their temperature will drop very low causing the player to possibly freeze once hit again with enough stacks of soaked, (here it is plausible that any skills or technique used by the player will be abnormally rushed as one would be in cold weather without suitable wear, this is quite far-fetched though so there’s no saying this is included ever or yet ).
In the example we went over wind magic lowering the temperature, other magics of course will have effects, for example fire of course raising the temperature (depending what the defensive player wields could have an effect on what this could mean, I imagine much later on once the player is capable enough to keep up their aura that magic interactions will take place, flame aura absorbing the flames, wind aura causing the fire to roar/ dissipate; could only be for defensive auras only too, I would imagine the aura should adapt to the players need, being defensive once players are blocking after cast or with it and so on )
Furthermore, temperature can have its effect on PvE elements of the game as well, some really cold ice? Melt this away with heat, ( unable to? Just bring some gunpowder barrels (weapon users especially), an ice or water user? crumble the ice to pieces. )
-another example would be the effect of seasons and temperature on food sources in-game too, what fruits are in season, you want to set fire to the hay? Go ahead, you may hear the farmer complaining in the agora and in need of assitance for other players to help undo the damage/ gather supplies.
And the list goes on… these are interactions that can be implemented bit by bit and PvP zones can be unaffected by most unless it’s realistic to incorporate such as the aforementioned combat interactions.
Seasons and weather can also have their effect for events as players already get rain and thunderstorms ( would be nice for events to return with much lower and rarer odds than before as remnants of the event and I mean MUCH lower odds, like seeing a coconut egg around egg collecting event time would be at least a hundred times more rare than it was before otherwise more and more rarer the more the event passes. )
Another idea was player literacy of the unknown language of old; this could just be part of a quest line though or actual skill that doesn’t necessarily have to be experience-based and could be part of the main storyline actually, where that npc helps decipher a text and then can be returned to for more quests apart from the main storyline leading to a unique item like the shell island village gift.
I digress, this idea does not have to added all at once, however could be added gradually so that the player-base has time to get familiar and give feedback on the gradual implemented pieces of this idea.
This would get the player more invested in considering more and more elements to factor in for their gameplay, for their sprites build and so on.