I would think rain would effect the range units effectiveness a bit, lets say you got a bunch of archers or something. The rains effect would be minimal, but it would still make it more difficult for the archer to hit the target. Imagine if musketeers firing in the rain, their munition gets wet and the overall effectiveness of the weapons are rendered pretty much ineffective in that kind of weather.
Things like tornadoes and stuff are things that don’t belong in AoE IV, but having some luck based elements to the game would make things a little unfair.
I would imagine farms would be inoperable in the winter, which is what would make ranching a little more viable option in winter maps and give players a reason to build a barn or something and raise animals.
Another twist they could add is frost, which kills all food production for thirty seconds.
What if the seasons gradually season change during a match, like every thirty minutes?
What if there was a plague going around and you had to put down your infected settlers before they all got it?
There is just so much untapped potential, sounds like chaotic fun to me, but not for most boring RTS fans who like fair balanced game play.