Scoobies as Strawhats:

Buffy is Zoro, an incredible warrior whose fear of being a burden to her friends leads her to hide her real pain. Horny, but only for deadly things. Keeps sacrificing herself for the crew.

Willow is Chopper, a sweet little nerd who deserves better and sometimes turns into an uncontrollable monster. Gets a real glowup after the timeskip.

Xander is Usopp, a real goobery coward with jokes that functions as the heart of the team. Weak and kinda bitchy. You knew this was coming.

Cordelia is Luffy, unrestrained by common sense and driven purely by ego, capable of speaking tremendous unfiltered truths to anyone she meets.

Giles is Robin, the researcher, nose in a book, quiet, but hiding a dark past and posessing tremendous fighting ability. Likes watching those dumb kids run around.

Wesley is Brook: thin, wears a suit, deeply horny. I have nothing else to say on this.

Spike is Sanji, because he's love's bitch, but at least he's man enough to admit it.