Again you should avoid anything that has to do with RNG, randomness is something quite difficult to dealt with for anyone trying to build a competitive team.
Either way, when trying to decide how long should the effect last you may have to ask yourself how long they need to be impactful while not overstaying their welcome, if they last for too few then it may be hard to take any advantage from them but if they last for too long then you may not see most of them and the early weathers may be the only important ones.
I find it hard to come up with an idea for turn 0 that does not involve RNG. Personally I think that always starting with the same field conditions (say Rain, Grass, TR) pretty much encourages everyone to bring a dedicated lead. This is not a problem necessarily, but it restricts building freedom to me. I think if only turn 0 is randomized, then the rest of the game is predictable with to rotations which also opens up strategizing.
Regarding the number of turns before rotation: also hard to say, especially without being able to test it. I think that 3 turns is close to ideal, which means that if you actively pivot your say rain sweeper in on the turn before rain goes up, you have a maximum of 3 turns to use it, but in most cases it will only be 1 or 2 turns. I would expect this to be both impactful enough to abuse the field conditions, but also not too overstaying that teams instantly get swept. With 3 turns of each field condition, I even think this meta could see an increase in the use of Protect, which could be interesting.
Lastly, I’m not sure on whether all weather, terrain and dimension should change at the same time after 3 turns, or one changes per turn in a set order. Ideas?