I want a green or purple haired girl before a blond, unless the blond turns out to have gold-tinted hair, then it's perfectly welcome. I'm not really into the blond-blond girls unless it's Domino or something.

I'd like to see a girl who is either
1. A total snob to Ash
2. A newbie or semi-new trainer who is convinced she is better than Ash
3. A PokeOsaka. ex: asks random questions and thinks a lot "Most of the Pikachu I've seen are yellow or gold. How come no one ever dyes their Pikachu's fur?" or "Why doesn't Brock drive a car?"
4. Similar to 1 only 4 is more reserved

Though, although many people think otherwise, the next gen trainer may be grass oriented (therefore green hair is likely ^_^).
May - fire
Hikari - water
next girl - grass?