Like Wapol vs. Queen Marilyn, Rob Lucci vs. Keiko Yamazaki, Yuki and Miki (the artificial nurses from Pure Trance) vs. Trafagal Law and Eustass Kid.
think about it, the possibilities are endless! and the opening song has to be We Are! or Kokoro No Chizu. I don't care who does it, I just want to see some thing like that so badly! XD