I would only watch this show if it was live-action. An animated would be horrible. Actually, I take that back, I'd watch a CGI show, they could do good with that.
Now though, with how much we can do with special effects, this show could be seriously awesome.
If I had to cast it, I'd go with unknowns, personally, but if I had to choose right now I'd pick:
If I was going with an older teen (16-18) series:
Jake: Thomas Dekker (Heroes, Sarah Connor)
Rachel: Kristen Bell (Veronica Mars[She'd probably be too old though]), OR Kirsten Stewart (Twilight, Jumper[again probably too old, but she could pull it off with some good hair dye])
Marco: Josh Peck (Drake and Josh [DON'T TEASE, lol, if you've seen how he looks now, he could totally pull off a Marco, he's even a good actor, so I think he'd be perfect for it.
Cassie: Someone fill this one in, I'm stumped.
Tobias: Michael Cera (I CAN'T STAND THIS KID, but he does pull off the whole loner feel, and he's funny, so he's up there[he'd have to grow his hair out though, my Tobias has long hair]), OR Jason Dolley (Some dude on some Disney show my sisters were watching, he looked the part, and I think he could pull of Tobias).
Ax: Well, this'd mainly be voice, and the human Ax wouldn't really have to have to same voice, so I think for voice, I'd pick Nicholas D'Agosto (Heroes), though I think he could pull off human Ax too... Dunno, but his voice is how I picture Ax's thought speak.
Hmmm, that's the main cast...
For Visser Three's voice: Chuck Norris (I'm KINDA kidding, but seriously, you KNOW you can picture it! And imagine what'd it would do for ratings!)
The Hork-Bajir voices: Whoever did the Covenant voices in Halo would be perfect, cause that's how I picture their voices.
Ok, I'm spent, I'll think of more later.