Final Fight: Seven Sons is the name of a cancelled game in the series. The game was first shown to the world in 2008 when Unseen 64 uploaded a video showing off an early version of it. It featured cel-shaded graphics and seemed to play like an updated version of the original, the camera was set on a cambox principle that imitated a rail-cam system of the old games. From what's been shown, there were a wide range of moves to do and a new "rage" mode. By attacking enemies, the rage meter fills up, when it is full it can be activated and makes your character stronger, larger and able to beat up enemies a lot more easily for a short period of time.
Capcom's Studio 8 were developing the game. But apparently after some of the higher-ups at Capcom saw it, they said that the game was not suited visually for its core audience. When the development team received this feedback, they apparently misinterpreted it into basically "this sucks, stop it". This led to Final Fight: Seven Sons being scrapped. Instead, the team went in a different direction, with Final Fight Streetwise being the end result. Streetwise was poorly received by critics.
Whether or not we'll ever find out anything more about this cancelled title is a bit of a mystery. There is currently one video available showing gameplay footage. The one playable character shown is supposedly Dean, who was from Final Fight 3. The character in the video does resemble him somewhat, and uses electrical attacks a few times.