Back in 2006, before Street Fighter IV went into production, Backbone entertainement made a proposition to Capcom to make Street Fighter IV, they even went ahead and created a bunch of concept arts, 3d models and what not.
The concept behind the “Flashback” had various meanings. The single player mode was set in Ryu’s flashback to SF2 and the story would have followed classic Street Fighter moments and revisit familiar characters. Also, there was meant to be a new game mechanic called “flashback ” that let the players to rewind time to undo mistakes.
The concept arts of Street Fighter IV Flashback were made in collaboration between Backbone, Massive Black and UDON (that in the end worked on the SSFII Turbo HD Remix arts). Unreal 3 would have been used as the main 3D engine for the single and multiplayer modes.