So this is more of an idea based off of 5Ds with a twist. So what if after Jack had left Satellite to go to Neo Domino, Yusei started to lose his cool and grow angrier and angrier slowly. That rage and anger started to slowly grow and one day, he completely lost it, a side that only Jack and Crow slightly knew about but didn't know enough about. He transformed into a Stardust dragon about the two times his normal human size. He didn't have control of this side, and many got hurt until he was caught and tranquilized by Sector Security.

It can go many ways, and this is kind of based off a weird dream I had.