The most believable rumour I've heard is that Willow is the season's Big Bad, her love of the black arts gets a little too black and it consumes her echoing the XMen's dark phoenix sage. In the season finale she's too powerful for Buffy to stop any way but the final way and Buffy ends up having to kill her own best friend.
This also seems to tie in with the "one of the regular cast leaves" rumours that are flying round the net.
This also seems to tie in with the "one of the regular cast leaves" rumours that are flying round the net.