A new world raid boss would be cool. Maybe even world raid bosses spread throughout the zones of all expansions. A giant Magnataur in Dragonblight or something, just for fun, or a super Fel Reaver in Hellfire for level 90 raids.
An 8th Sha, however, is impossible. There were only 7 heads of Y'shaarj, and there was a corresponding Sha for each head. That is only further solidified by the 7 Burdens of Shao-hao. And then, on top of that, the Sha are defeated along with Y'shaarj's heart (as far as we know), meaning that even the established Sha cannot manifest any more.