Ed Boon's latest teaser tweet suggests a major character reveal tomorrow.
It seems that this is the big news that Boon teased the other day. Today, Boon made a tweet heavily implying the revealing of more characters tomorrow.
— Ed Boon (@noobde) September 2, 2015
Four silhouettes can clearly be discerned, although speculation beyond that is difficult. Mortal Kombat 10 has already welcomed four new characters through its Kombat Pack: Jason Voorhees, Predator, Tanya, and Tremor. It seems unlikely that new DLC characters will be included in this pack, which explicitly promised 'four extra characters'.