This is according to NetherRealms’ Ed Boon who has been making various tweets that all seem to hint at Spawn being a possible guest character for the series. It started with Boon posting a survey in which he asked which non-Mortal Kombat character would make for a great addition to Injustice 2, in which the majority of voters seem to have chosen Spawn.
Following the survey, Boon continued to tweet about Spawn which has led some to speculate that the character will indeed be a guest character in Injustice 2. Of course we cannot confirm this yet, but all signs are pointing towards that possibility. NetherRealm in the past has been pretty good with including interesting characters in their games, like bringing Predator and Jason Voorhees into Mortal Kombat X, so the idea of Spawn finding his way into Injustice 2 isn’t really that much of a stretch.
That being said, there is no release date set for Injustice 2 at the moment, but it is expected to see a release some time in 2017.