Okay trawl time!
Chance clauses: Let's just explicitly specify a default and suggest ways to circumvent that default. Eg. Default to check at the start of action, before any mon moves, and if you want to check at the time your mon acts, write a "when your Pokemon acts" phrase. I stated the example above and not the other way around because I personally like giving players an easier time to remember, and that seems easier to me, kind of. Moving to Discussion because we really need to resolve this before it goes about ruining play experience, Jesus Christ (and I'm not even Christian).
Future Sight: I like Frosty's starting point. I'll move to Discussion using his suggestion and EM's questions as starters, then see if people came up with more obscure interactions that needs to be clarified.
Terrain and Camouflage: By default, arena mechanics get overwritten by items get overwritten by moves, in that order. So if you use Camouflage or Nature/Secret Power on a Rock Gym that's under Electric Terrain and a Heat Rock boosted Sunny Day, go with Terrain and Sunny Day. Nothing in EM's post warrant discussion for changes.