I believe Kai will be brought back into Mortal Kombat either in this game or the next. I know that the Mortal Kombat Development Team has not forgotten about Kai.

Kai left Mortal Kombat 4 on a quest for enlightment. I think his quest could be interrupetd due to the Outworld threat of the Dragon King.

Kai was once a Shaolin Monk and I think the reason he could come back into Mortal Kombat is to help Raiden and Kung Lao in their quest to battle the Dragon King.

Kai would return only to fight the corrupted Liu Kang and he would wish to kill his demonic friend so that his soul may rest in peace. Kai would journey with Kung Lao in Outworld as they search for the evil Liu Kang.

I think the MK4 ending for Kai didn't happen - I mean Raiden still has his staff (another words he's most likely in the Prison Stage or playble, or he'll be kept for MK7 - cuz a lot of people have been talking abt. him (even though there r some haters out there; the majority like him)