I can't keep up on this thread o______o There are three ways Misty could confess:
1. Accidentally while arguing
2. When Ash is sleeping (Or she thinks Ash is sleeping but he's really not and he hears it 8D)
3. I thought it could happen after some kind of a party or something, maybe in the end of the series when Ash has become a "Pokemon master" (if he will). Somehow like this:
Ash: That was a great party, huh?
Misty: Yeah, but what're you gonna do now that your dream has finally come true? *yawns very tiredly (is that a word?)*
Ash: I'm not sure... I have to think about it.
Misty: *almost asleep, leans onto Ash* You're finally back...
Ash: *blushes* Misty?
Misty: *mumbles* ...I love you... *asleep*