The redesigned appearance of Cinder for the new Killer Instinct was originally going to look far more alien than how he appears now. In the original concept art, he had two sets of eyes and two thumbs on his hands. However, they felt that taking Cinder in this direction would be "betraying the original concept in favor of convenient setup for the sake of a rivalry with Glacius". Thus, his design was changed to be more similar to his original appearance in the first Killer Instinct.
Chief Thunder is the only character which doesn't have his classic look as default for his retro outfit, This is likely due to his retro outfit from Killer Instinct being considered offensive to the Native American image.
For TJ Combo's dynamic theme, Mick Gordon, the composer, purposely made it the longest theme in the game so far, with all the variations. The reason was to give a challenge to YouTuber RKosmik, who makes videos by compiling all of a single stage's theme variations into one single track. TJ's theme clocks at around 20 minutes, where the other character/stage themes are around 8 to 10 minutes long. Surprisingly enough, RKosmik accepted the challenge and compiled the theme.
When both characters stand still/idle for a period of time (around 30 seconds), the stage music will change and play a remixed version of the character/stage theme from either Killer Instinct or Killer Instinct Gold. This is only noticeable when using a returning character, such as Jago or Orchid. For newcomers, like Sadira, Kan-Ra and Aganos, the music will instead change to a remixed version of the main Killer Instinct theme, but flavored according to the stage theme. For instance, Kan-Ra's variation will have an Egyptian flavor, while Aganos's variation will have a Greek flavor.
According to composer Mick Gordon, Spinal's theme was a global effort that combined the talents of twenty people across five countries over three months. The "Spinal Choir" consisted of thirteen men singing in Swedish in Gothenburg, the lyrics were written by Mick Gordon and Pontus Rufelt, and the brass was recorded and arranged by Hugh Davies in Guildford, United Kingdom.