Tanya - ninja-ish yellowy female counter to Scorpion - what's not to like... The game needs more black of skin types, bad girls are cool and I quite like her demon origins and move sets, I think she could be done very well in MK9.
Fujin - out of the many characters I've looked up on the MK wiki, his design and move-set seem most unique, plus they made Rain great, they can do the same for Fujin.
A new character, maybe something based on Tremor, or maybe someone new entirely.
Definitely not playable bosses, sorry to those who want that, I just like the bosses to stay that way and not have to fight them online.