More seriously, if Oda ever does do a serious romantic pairing (Luffy x Boa Hancock doesn't count) I'd like to see Luffy x Nami or Nami x Zoro (the latter just amuses me).
Generally speaking I tend to stick with Canon pairings, or hinted at Canon parings.
Emma x Hook (once upon a time)
Hardison x Parker (Leverage)
Haruhi x Tamaki (Ouran) < I was rooting for this one long before it happened.