I think interactive weather and AI acting accordingly is not impossible but i fear it's not at this point at least, very high on their priority list.
It's not impossible but it's somewhat hard to achieve - especially if you want something more detailed and balanced.
I know this because I created a mod months ago and didn't release it because of obvious reasons.
In order to give an idea of what it does:
- Based on the calendar date, there is a probability of weather actions happening when you go into battle. i.e. if you are in desert and if it's summertime, it's likely that you will see a notice saying that there is a heat wave. Or if you are in Vlandian grassland and if time is Autumn, it's likely to rain more. I will explain what they all do below.
- Each soldier had an adaptability value for the terrain they are standing on. Starting value was based on where they are standing and what their culture is. Meaning that: if you have 5 Sturgian standing on Aserai desert, they are 100% not adapt to heat and desert and they will fight extremely horrible when you go into a fight with them. But each day, they gain adaptiveness point to desert and heat and after 2 weeks, they are fully adapted to desert and fights exactly same as they fight in Sturgian snow. But now they are fully adapted to desert, if you suddenly go to Sturgian mountains, they will again need a week to adapt back to snow.
- Each weather and weather event has a direct impact on soldiers regardless of their adaptation. This is very initiative and guessable things actually. Like, if your soldier is very heavy, carrying a huge battle axe, and has thick armor, it's very likely that it will fight slightly worse than his best because of the Aserai desert heat. Or if your archer is too light, it's very likely that they will shoot inaccurately because of the shaking hands in Sturgian Snow. Same goes for cavalry too, if it's raining and if terrain is grassland, it's likely that horseman will move very slow because of the heavy horse armor and the weight of person it's carrying.
I know perhaps all these stuff sounds like impossible to you but it was actually working fine. I have seen Vlandian armies getting defeated in Aserai desert because of the heat, even though they had man and armor power advantage. Or I have seen Imperial cataphracts getting beaten by Vlandian sharpshooters because rain made catapthracts too slow ( and drifty, yes that's a thing ) But it's very had to keep the balance because I realized that Sturgia and Aserai have great advantage with their location - it's hard for others to conquer there but it's easy for them to conquer their lands because they are getting less effect from the weather etc
At best what you will see from the Vanilla game will be similar to Warband. You will see horses getting slower on rainy day and that's probably all. They already have seasons in-game but they just have to fix snow and rain dropping. The game already has both. In rain especially if you increase intensity it also shows lightning and thunder. Snow is also already in the game. I guess they had an issue with rain since from Beta - same for fog as well. Only works in a few maps. It's causing instant crashes on others.