In Kanto Misty and Brock were more than good enough traveling companions imo, being there to get familiar Ash with pokemon basics and helping him to grow as trainer.
Granted like some mentioned Sabrina might have been intriguing choice, but whole psychic powers thing and fact that she was older would probably mess up story too much imo. Still i wish we got to see her more often because whole personality distorter issue had interesting premise behind it.
However in Johto i feel addition of Marina as second female traveling companion would add something refreshing to main cast and set up ground for interesting character interaction having two girls for first time in one group. That concept could have been used in theory in Hoenn too if Misty joined them in BF and May stayed, but alas.
Only issue with Marina in Johto emerges from fact how contests still didn't existed at that time ,so character would be prone to become quickly stale without some purpose behind his existence. That's unless writers came up with anime exclusive goal which would be plausible enough to support her stay in cast.
Leaf had some promise to become traveling companion to Ash and co having every predisposition to take up role as coordinator. But i guess this was never meant to happen simply being blew out from competition.
In Sinnoh i think Reggie would have been something worthy enough to be taken in consideration. Not necessarily by replacing Brock, but still whole breeder thing would allow for better character dynamic with Brock having someone to relate too and vice versa, preventing to end up so much left out from group dynamic at times.
In Unova im satisfied with cast so far and wouldn't change anything about it, but in 6th generation it would be nice way to shake things up to see one of older traveling companions back in cast for awhile again, to get update and sequel to his story.
I can't help but think marina would have made a good misty replacement after OI, and that lyra and her breeder sidekick would have made a better addition to the main cast if they replaced brock for the last 40 eps of Sinnoh. As for BW, I would have liked to see an older Ash and Hilda.
If Misty left before Johto that would be incredibly short run for female sidekick, and i don't think i would welcome it. Especially when majority of her growth didn't even happen until Johto and hosos.
That being said, speaking of other Marina they met in OI i always thought she would have been great rival for Misty in Johto.
her powers would come in handy!
