I have also read somewhere that Quan Chi and Shinnok have made appearances in this. Quan Chi is a free - roaming evil sorceror, similar to Shang Tsung. His powers are great, and he has made an appearance in one of the recent episodes of Defenders of the Realm. snakeLike Shang Tsung, he can morph, to some extent. He has a combination belt/cane/snake, which he uses as his primary weapon. It is Quan Chi's picture on the side of the MK4 arcade machine. Shinnok is an elder god, who was banished into the Netherealm. Only a special amulet could free him from his imprisonment. Noob Saibot, a shadow warrior who made appearances in MK2, MK3, UMK3, and MKT, joined Shinnok's special forces in trying to retrieve the amulet, which was hidden under a Shaolin temple (appropriately called "Shinnok's Amulet"). Shinnok will be the main boss in MK4 (he will replace Shao Khan), and Noob Saibot, who retrieved the amulet, will be given newer, greater powers and thus he will become a sub-boss, similar to Kintaro and Motaro.

Indeed, Shinnok was originally set to appear as outlined in the Writer's Guide, with the excerpt below and therefore confirms this rumor.

From the Writer's Guide:

SHINNOK'S RETURN:

On the dead planet known as the NETHEREALM, a fallen elder god learns of Earth's failing dimensional fabric and the vast gates to other realms that exist within the planet. He realizes that not only is this a perfect time for an invasion, but it's a perfect time to avenge his past.

The fallen god is known as SHINNOK, and he plans a fierce takeover of Earth. Leading his minions of LOST SOULS - THE BROTHERS OF THE SHAOOW - Shinnok attacks. Our heroes immediately respond and begin to drive the unholy warriors back through the portal. However, Shinnok's plan is two fold, and he pens another portal directly behind our heroes, driving them into the Netherealm.

On the Netherealm, our heroes must confront a whole army from the Brothers of the Shadow and figure out a way to get back home.

With our heroes trapped in the Netherealm (less Nightwolf, Kiva and Rayden), Shinnok plans to destroy the Earth. His scheme begins with disposing of his arch nemesis Rayden. Shinnok sets a trap for Rayden and plans to force the thunder god into revealing the whereabouts of the group's headquarters. However, Shinnok's plans has some holes, for he didn't realize that Nightwolf was in the headquarters working on obtaining the location to the Netherealm.

When he finds it, Nightwolf enters the Netherealm with Kiva and tracks down our heroes. He helps them escape and they seal the "rip" in the portal behind them.

Meanwhile, Rayden falls into Shinnoks trap. Rayden is forced to face a past that he wished to forget. It was a past filled with untold treachery and deceit and it ended up with Rayden banishing Shinnok from our known Universe.

Back at the headquarters, our heroes realize what's going on. With the blazing speed of their Dragonjets, they're able to save Rayden and help him battle Shinnok. They defeat him and send him back to his own world, for good.