My best friend asked me if I'd date her when we return for school next year if she were slimmer and more tan?

It could be both. but i would think that she probably wanted you to say something about how you already liked her just the way she was...without slimming and tanning. girls will do those sort of things just to see how you respond. Like they'll say something negative about themselves, usually small things, to give you the opportunity to turn it around into a positive. i can understand how the flirtatiousness can make a person harder to read though.