Forgive me for returning to an earlier question, but this is my first foray into the CAP process and I wanted to sample other responses before.
4.)
I think the ability Forecast (especially when coupled with access to Weather Ball) has strong, underutilized abuse potential. Form change to match weather (supposing strictly a type change) could allow an appropriately-equipped pokemon to act both reactively and proactively.
Reactively, for Rain, Sun, and Hail, the corresponding type gained by Forecast is resistant to itself (i.e. Water type resists Water attacks, etc.). This means that a pokemon with Forecast (supposing it doesn't sport abysmal stats a la Castform), can be used defensively in response to a weather-based sweeper, and can do so reliably for multiple weather types. This ability to respond to sweepers can be honed if a pokemon had a secondary typing in addition to its Forecast typing (for example, a Grass-[Forecast Type] pokemon would resist water attacks in Rain even more strongly, while only neutrally resisting fire-, rock-, and ice-type attacks in their corresponding weather). The ability Could 9 here would act similarly if given a typing biased against a certain form of weather.
Proactively, a pokemon with Forecast could, especially with access to Weather Ball, become a versatile and powerful attacker, able to fit on various weather teams, as it could reliably gain STAB on four different attacking types. With Weather Ball, there is even a sort of click-and-forget sort of thing that could be seen, as even supposing a pokemon switched in to change the weather you were trying to abuse, you would still get off a STAB, potentially boosted attack in the new weather to punish them for it. The great disadvantage to abusing Forecast this way, of course, is that you either: have to have another pokemon set your weather for you (acceptable), operate strictly reactively (pretty high variance), or waste momentum and a moveslot setting your own weather (terrible).