Killer Instinct was one of the release titles for Xbox One back in 2013, a 'free to play' fighting game that managed to become very popular thanks to good communication with the fighting community and really good gameplay.
Despite reaching over ten million players, the support eventually ended in 2017 and since then, we haven't got any sign of life from the series. Fortunately, this might be about to change, if the insider Nick Baker from the XboxEra podcast is to be believed. He claims to have sources explicitly claiming that a new Killer Instinct is "in active development".
Killer Instinct from 2013 was originally developed by Double Helix Games, but later moved to Iron Galaxy Studios as the former studio were bought by Amazon. Baker says he doesn't know which studio that is making the game, but that it's neither Double Helix Games or Iron Galaxy Studios, and that he does not believe it's NetherRealm.
Hopefully we'll hear more about this in a not too distant future if true, but until them - remember that it is just a rumour.