My turn.
Also I think it would be cool if the song is not completely mashed up, but every person adds on to the end of the previous person's arrangement.
For example on a song timeline, 1 minute in is Kita's arrangement, and then the second minute is in my arrangement, and so forth (basically not adding parts on top of other parts).
Therefore someone doesn't have to do all the balancing/mastering for the entire mash-up, and it will sound more solid

EDIT:
Done. Added some synths and kicks and a sidechain to the kita lead.
http://www.mediafire.com/?szttjnjw1cl