Fighting Force, although similar enough to Streets of Rage in its final version, was originally straight up Streets of Rage 3D. The original character models were more influenced by the series, and even shared the stat ratings. Interestingly enough, two of the bosses in Fighting Force -Vulcan and Hammer- were originally intended to be playable characters.
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While the combat's the same, the environments were a lot more colorful. I'm not sure whether the eventual change was due to graphical limitations, or an attempt Core made to distance themselves further from the license. Fighting Force is still perfectly good, but I wouldn't have complained about playing a more colorful version with better music, either.
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As far as I can tell, all links to the playable beta are dead, but there are still videos lying around.