A listing on Newegg and eventually Amazon confirmed that Killer Instinct Definitive Edition was a thing and it is set to release this September.
Killer Instinct: Definitive Edition includes all 26 characters with 20 stages, Killer Instinct Classic 1 and 2, the original soundtrack, and a bonus skin pack. There are also behind the scene videos and the soundtrack is included on a separate disc. Check out the listing on Amazon with high res box art and more details here.
Killer Instinct is available on Xbox One and Windows 10 and the Definitive Edition releases on September 20 for $39.99.
Source: God is Geek