My suggestion is to create a concept and usage of Enton/Blaze Release within the RP that is independent from Amaterasu, Kagutsuchi and the Mangekyo. My reasoning for this starts with the distinction between Amaterasu, Blaze Release, and Kagutsuchi.
It is a common belief that Blaze Release and Amaterasu are the same thing, that Blaze Release is the manipulation of Amaterasu and that's it. This, however, is a misconception. Blaze Release is not a jutsu, it is a Nature Release. The fact that it ends with the word "Release" is proof enough. It's Japanese name is Enton. The suffix, -ton, is used in the series to indicate a Nature Release. Enton is not exclusively the manipulation of Amaterasu, but one usage of it. The manipulation of Amaterasu is Kagutsuchi, a jutsu that is used by channeling Enton through the Mangekyo. The black flames are spawned through channeling Katon through a Mangekyo(The databook and manga classified Amaterasu as Katon). However, Katon is not limited to only being used to spawn black flames via a Mangekyo. Similarly, the black flames are controlled by channeling Enton through a Mangekyo. This does not have to be the only way to use Enton.
There are possible issues with this concept, but I don't think they hold up. One issue is that such an ability within the RP would have to be completely made up and original, created specifically for the RP. This isn't a problem, and has been done before. The most notable example is Yin-Yang. Before Y/Y was revealed in the series fully, the RP had its own version for years. This shows its not against or ridiculous to create usages completely new for the RP. It was revoked because the real explanation was given, I believe. Blaze Release wouldn't encounter the issue of a real explanation being given that warrant it being revoked because the series is over.
The second potential issue would be how exclusive it can be. Some may argue that, logically, it should only be for Sasuke or Uchiha because that's the only category and character to explicitly display it. This is resolved because, as Cali once told me, it's not truly about whether or not it goes with the manga, but whether the staff and RP wants it. Thats why, despite only one person showing Ice Release, Scorch, Dark and Dead Bone Pulse, multiple custom characters can posses and use these abilities. The same can be applied for Blaze Release.
How would it work? Well, Blaze Release would be the combination of Raiton and Katon. Both elements have shown that they can be manifested as condensed, solid forms or semi-tangible streaming forms. Lightning takes this even further by appearing as completely intangible, just being a bright light. Sasuke has also shown shape manipulation with lightning that goes beyond what has been shown for shape manipulation with fire. By combining the ability to shift the densities and nature of these elements, you'd achieve the ability to do so on a level beyond what you could do just manipulating either one. I believe this is how Kagutsuchi works, using the advanced shape manipulation potential of Raiton and combining it with Katon to enhance the shape manipulation potential of the latter. That's why Sasuke can manipulate the black flames.
My suggestion in the RP is making those who have Blaze Release gain the ability to remotely control their existing flames. Techniques of Blaze Release would allow one to control the density, brightness, size, shape, temperature, etc. of their Katon jutsu after they create them.
It is a common belief that Blaze Release and Amaterasu are the same thing, that Blaze Release is the manipulation of Amaterasu and that's it. This, however, is a misconception. Blaze Release is not a jutsu, it is a Nature Release. The fact that it ends with the word "Release" is proof enough. It's Japanese name is Enton. The suffix, -ton, is used in the series to indicate a Nature Release. Enton is not exclusively the manipulation of Amaterasu, but one usage of it. The manipulation of Amaterasu is Kagutsuchi, a jutsu that is used by channeling Enton through the Mangekyo. The black flames are spawned through channeling Katon through a Mangekyo(The databook and manga classified Amaterasu as Katon). However, Katon is not limited to only being used to spawn black flames via a Mangekyo. Similarly, the black flames are controlled by channeling Enton through a Mangekyo. This does not have to be the only way to use Enton.
There are possible issues with this concept, but I don't think they hold up. One issue is that such an ability within the RP would have to be completely made up and original, created specifically for the RP. This isn't a problem, and has been done before. The most notable example is Yin-Yang. Before Y/Y was revealed in the series fully, the RP had its own version for years. This shows its not against or ridiculous to create usages completely new for the RP. It was revoked because the real explanation was given, I believe. Blaze Release wouldn't encounter the issue of a real explanation being given that warrant it being revoked because the series is over.
The second potential issue would be how exclusive it can be. Some may argue that, logically, it should only be for Sasuke or Uchiha because that's the only category and character to explicitly display it. This is resolved because, as Cali once told me, it's not truly about whether or not it goes with the manga, but whether the staff and RP wants it. Thats why, despite only one person showing Ice Release, Scorch, Dark and Dead Bone Pulse, multiple custom characters can posses and use these abilities. The same can be applied for Blaze Release.
How would it work? Well, Blaze Release would be the combination of Raiton and Katon. Both elements have shown that they can be manifested as condensed, solid forms or semi-tangible streaming forms. Lightning takes this even further by appearing as completely intangible, just being a bright light. Sasuke has also shown shape manipulation with lightning that goes beyond what has been shown for shape manipulation with fire. By combining the ability to shift the densities and nature of these elements, you'd achieve the ability to do so on a level beyond what you could do just manipulating either one. I believe this is how Kagutsuchi works, using the advanced shape manipulation potential of Raiton and combining it with Katon to enhance the shape manipulation potential of the latter. That's why Sasuke can manipulate the black flames.
My suggestion in the RP is making those who have Blaze Release gain the ability to remotely control their existing flames. Techniques of Blaze Release would allow one to control the density, brightness, size, shape, temperature, etc. of their Katon jutsu after they create them.