Dreamy

Anticipation only works when there's a super effective move or a move that can OHKO iirc. So if Cloud Nine is too situational than Anticipation would be too

It's not that cloud nine is too situational, its that it wouldn't have time to have any effect. if it actually removed the weather it would be good.

I didn't really make it clear but I'm looking for abilities that would have an effect on the battle if on a pokemon that was switched in after a KO and then mega evolved on turn 1, other than the ones in the original message. 

Also sadly while forewarn, anticipation and frisk all technically count, they're useless to an AI trainer.

Magic Guard is the only other one I've since thought of as it would protect from any hazards on switchin

Imposter,damp,download and the auras I suppose. Does flower gift,friend guard, queenly majesty and the stat preventers (leaf guard) count? What about lightning rod, flash fire and storm drain?