I'm gonna bring some flavor to the table here.
I'm only doing the main cast right now. Here goes.
Ash would honestly be a Chimchar. I'm going to explain why. Ash has a fiery battle spirit, and very childish (since he's ten, come on now). Like Chimchar, Ash has potential to grow into a better trainer/Pokemon, since Chimchar evolves into a Fighting type, after all. Feel like Ash would fit very well as a Chimchar, since his potential would mostly be in his Blaze ability, lol.
Misty would probably a Piplup. Misty is kind of plucky, and stubborn. She also is very tsundere, kind of like Piplup. Since she wants to specialize with Water types, the Pokemon clicks, in more ways than one. I do feel like she'd evolve later into a Prinplup, and be even pluckier, to be honest, however, I don't think she'd evolve all the way into Empoleon, just a thought there.
I put a little thought into Brock, and I honestly had a hard time choosing, but Purrloin. I'm going to explain why Brock would become a Purrloin. Purrloin are known for their Charming looks or something of that sort, in the Pokedex entry, their deceptive good looks. Brock is flirtatious. However, he wouldn't know the move Captivate, which would make the girl Pokemon not notice him at all. I had a tough time thinking of what Pokemon Brock would be, since nothing really came to mind for him until last second.
Tracey is on of the easier ones. I cam to the conclusion of Smeargle, since he's a Pokemon Watcher, and is shown to be quite the artistic person on his own. I don't have much to say for this one, since Tracey barely travelled with Ash for me to have a full synopsis on him, so that's where I'll end this one.
May would be an Eevee, for similar reasons to Ash. In the anime, May wasn't sure what she wanted to do in the beginning of her journey, like how an Eevee has many options for evolution. The idea is similar in this regard. Since she was a new trainer, and all. Eevee has potential to grow, and choose an evolution to perform as. In regards to evolution, unlike the Eevee she had, I feel like May would maybe evolve into a Flareon because of her determination to be top coordinator, lol.
Max, on the other hand, I guess would be an Eevee as well, since he's related to May, I'd think he'd be the same type of Pokemon. I have nothing else to say about that.
Dawn, hmm. She'll start out as a Feebas, but will eventually become a Milotic. Dawn is all about looking good/beautiful, and showing off her Pokemon. She started out pretty bad, thus why I feel like she'd turn into a Feebas, which is a shabby Pokemon until it evolves. While I'd love the idea of her being a Shiny Feebas, that'd ruin pretty much all the fun, but I'd like the idea if she were a Shiny one! Aha. (Someone on Twitter pointed out Milotic to me, I didn't come up with this one.)
I'm not entirely sure on my answer for Cilan, but he would probably be a Munchlax. He's a connoisseur, after all. While I feel a Big Eater Pokemon isn't a good idea for him, I can't think of any other Pokemon that'd actually work for his character, unfortunately. He would probably stay a Munchlax. I can't and don't want to imagine him evolving into a Snorlax, yikes.
Iris would be a Swablu. I picked Swablu for the Pokemon she becomes since she's an aspiring Dragon Master. Swablu evolves into a Dragon type, after all. I can see her pecking Ash and saying he's "such a little kid." Since she's shown to be a little... wild, I'm supposing she'd fly a lot as a Swablu. She wouldn't evolve into a Altaria, honestly, since her goal went nowhere.
Clemont would be a Porygon2. Clemont likes to invent things, and Porygon2 was a manmade Pokemon from scientists. Perhaps he would try to install new drives into his system that always fail, I could see that working for him if he were a Pokemon, not going to even lie about that. I honestly don't think he would evolve into a PorygonZ, however.
Bonnie would be a Porygon (yes, I know, bad idea, I'm sorry), since she's related to Clemont. Not much else to say.
Serena would be a Fufrou. Serena changed her entire appearance after losing a Pokemon Showcase, and the Pokemon on its own would be good use in performances, so I'd imagine she'd definitely be good as this Pokemon. She'd only change her coat every once in awhile, but the cut she would be after losing, perhaps she'd change her style to the Debutante Trim.
Finally, Jessie and James would both be Ditto, James would be Shiny so Jessie and James can be differentiated from one another. Well... since they're "masters of disguise", I could see these two transforming into various Pokemon to fool Ash and co. I don't really have much else to say in this regard, just that them both being Ditto works for me.
I'd go on to the rivals, but I already spent an hour on this post...
