Some players have theorised that the game was originally supposed to be and that it was changed mid-development, but there's more evidence against that theory than there is for it.
One of the most surprising things about Mortal Kombat 1 is how it managed to break the pattern that NetherRealm Studios had set up over the 🅷past de🎃cade. Ever since 2011, NetherRealm has jumped be𓂃tween two series, Mortal Kombat and Injustice, breaking up the new releases by swapping from one to the other. Then came Mortal Kombat 1, the second game in the series in a row, breaking that cycle for the first time in over a decade.
This change in release pattern caused some fans to believe that Mortal Kombat 1 originally started out as Injustice 3, and changed to a new Mortal Kombat game late in development. Over the past few weeks, , with some point towards the lack of features like a combo-pin🍨ning button and a general lack of skins as a sign that development had to switch gears fairly late into development.
, another reason (and arguably the biggest) why some fans think that Mortal Kombat 1 was supposed to be Injustice 3 is its first Kombat Pack. Alongside some Mortal Kombat characters (two of which are represented in the campaign), Mortal Kombat 1's first wave of DLC is adding three superheroes - Omni-Man, Peacemaker, and Homelander, all of which seem like they'd be good picks for a new Injustice game.
Although that's a decent point if you ignore how violent those characters are, there's actually more evidence that goes against the theory than there is for it. For starters, Ed Boon himself talked about the topic back when Mortal Kombat 1 was a♒nnounced, saying, "The thought was that if we did an Injustice game, it would have taken even longer and been seven years between MK games. So, we said okay let's do another Mortal Kombat game".
Some fans have retroactively taken that as Boon saying that the team started on Injustice 3, but it's pretty clear from the conversation that he was talking about conceptualisation and that the team simply started considering working on a new Injustice before moving on with a new Mortal Kombat. explaining how Mortal Kombat 1's development has progressed, and it makes it pretty clear that Injustice 3 was simply an idea.
To add to the evidence that Mortal Kombat 1 has been NetherRealm's key focus for a while, that says that Jean-Claude Van Damme's voicework was done during the pandemic and had to be done remotely, meaning that it was likely between 2020 and 2021, making it very unlikely that NetherRealm suddenly switched gears to Mortal Kombat 1 at the last minute.