I think a live action is not good for Pokemon. Mostly because the anime mostly known, and Ash Ketchum and it's plot don't make a good move.
Pokemon Special would make a greatCG animation, and maybe a good movie, having a GREAT luck with the director. I have no faith on Hollywood movies, because they always dumb down the Japanese and game productions. American movies and series have genericaly good image and photography, but not always good plot.
Other option, that i think would be great, it's a TV series, like Game of Thrones that take it's own vision, like the Pokemon manga, but with it's own interpretation and world building. With a more mature take, mostly a fantasy version of our world . It has potential to be successful and break the stigma that Pokemon is just Ash Ketchum and something childish. And of course, would be based on the games plot but with artistic freedom.
Considering the possibility of this hypothetical situation, which particular actors would you like to see portray your favourite characters? Looks wise, I think Ashley Benson or Rose McIver might be a good fit for Viola. Maybe AnnaSophia Robb for Serena? :S
The boy and girls will be the main characters of the series. Each season will follow a genararion. They will need to be aged. Otherwise, will be ridiculous to have 10 or 16 years world guys saving the world. They will be young adults, with no specified age.
For the main female characters, I would go with:
Leaf: Leighton Meester or Victoria Justice - I see Leaf as being pretty, daring but polite and nice girl
Lyra: Ellen Page or Lily Collins - I don't visualize her being particularly beautiful, but really charismatic and funny, to the point of being clumsy and hilarious (though I think Lily Collins is pretty, she can look like an avarage girl with customization)
May: Emily Browning or Levan Rabin - Both remember me May, specially Levan Rabin on Pierce Jackson... I don't know why... maybe because I think May is a casual and relaxed girl, still beautiful but not on a womanizer way.
Dawn: Camilla Belle or Emilia Clarke - Both are beautiful, and until now, Dawn to me is the prettier of the main characters, but she don't care or neither realize it. Also, she's really friendly and lighthearted.
Hilda: Kaya Scorelario or Kristen Stewart - Yeah, i believe Hilda is a headstrong girl, not girly, rebellious and vigorous. I know Kristen is not likeable, specially on Twilight, but I relate the actress with Hilda... Both have the same vibe.
Rosa: Saoirse Rowam or Dakota Fanning
Serena: Melanie Laurent - She's french, good actress, pretty, smart and low profile. Another choise is Lea seydroux. I can't see Serena as a pretty blond american girl. Taking account she's french, I think she's naturally pretty, but not a Barbie, and really smart and ambitious.