Ok. I'm back. Anyways. Misty has got to come back. She has to. And also, I doubt that she would be Ash's cheerleader if (and maybe when) she returns as a main cast member.
Also, who says that she wasn't a gym leader when she met Ash. She was one since she confirmed it herself in the Cerulean City Gym when she somehow appears out of thin air in the pool area.
And for all those people reading this who says that she can't leave be a gym leader since she travels with Ash. Look at Giovanni, he is away from the Viridian City gym for long periods of time leading Team Rocket (not JJ&M, the ENTIRE organization. since I hate Team Rocket {the organization, I mean} as a whole.) and he's still a gym leader and (offtopic) it is a wonder that he wasn't busted by the pokemon league. (end off topic remark) (Ok, that may be a bad example, but still.).
If she returns to the main cast, hopefully Daisy can be able to hold her own for quite a while. (I mean, she (Daisy) showed her loyalty to her Luvdisc (I think it's name was Casurin orsomething ) (both Misty and Daisy own one each, thus equaling 2) risking everything to get it back (from what I heard). And loyalty to a pokemon is a very important part of winning matches and being a pokemon trainer.)
Also, Misty has such an super-identic memory, she could remember to when she was a baby. (I think it was in cerulean blues [although I didn't watch it.].) I doubt that her sisters told her, since (from what I've heard.) in the beginning of the episode she still feared Gyarados before she faced it and caught it, too much to have been retold by her sisters.
Also, I know, it is insane. If it is one thing the writers hadn't noticed, it's that it is becoming exactly like what had happened during the "Orange Islands" fiasco.