Mortal Kombat X co-creator Ed Boon is teasing something on Twitter, and that something appears to be four more kombatants.

Whatever Boon is teasing could be confirmed soon, unless he's just having a bit of fun with fans. Famously, he has teased characters that were later revealed to be jokes.

Boon tweeted "Tomorrow?" along with a picture of four silhouettes. In front of those four silhouettes, there's one of the world's most chilling phrases: "Who's next?"

Tomorrow? pic.twitter.com/fMoXUSRvjf

— Ed Boon (@noobde) September 2, 2015

There are currently 29 playable characters in Mortal Kombat X. The Kombat Pack DLC brought the game up to 29 playable characters when it released Jason Vorhees, the Predator, Tremor and Tanya. Goro was released as a preorder bonus then later offered individually.

It's anyone's guess who the four characters are in the teaser image. On top of that, it may be more or less than four new characters. Again, this could be a misdirection or a bit of fun for Boon.

Anyhow, here are four Mortal Kombat regulars who might be in Boon's teaser image:

Noob Saibot—He has been in the game since the first entry. That's because he's the original Sub-Zero. He was killed in the first game then brought back as a wraith. Maybe he's been killed again, or maybe he's about to return to those willing to pay a little extra.

Cyrax—This cyborg has been a series regular since Mortal Kombat 3, but he/it hasn't appeared in Mortal Kombat X just yet. His pals Sonya Blade and Jax Briggs are in the game, however.

Smoke—He got his start as a hidden character in the first few games. He was made into a cyborg at one point, but then the storyline stalled, and he went back to being human. What hasn't changed has been the smoke that pours out of his pores.

Rain—This "purple-klad" kombatant was originally a joke, a reference to Prince's "Purple Rain." He was eventually released, but he served as evidence that Boon is willing to trick or treat Mortal Kombat fans.

Mortal Kombat X launched on April 7 for the PC, PlayStation 4 and Xbox One. There were mobile versions of the game, which released later, and plans for last-gen versions of the game. However, NetherRealm Studios and Warner Bros. scrapped plans for last-gen ports of Mortal Kombat X.