I've seen this done a few times, but it never has really gone anywhere. I believe the main reason for this is due to the fact that it requires the fan fiction to be a crossover to draw out the true potential of such an arrangement. The best two series to crossover with in this scenario would be DxD and the Fate Franchise, in my opinion, due to all of the various applications it can have in both worlds. I don't think Bleach is really all that suitable, unfortunately, because unless the world gets expanded via a crossover with DxD, Bleach is a pretty isolated kind of series that I prefer Ichigo to be the main character of without the kind of interference Kirito can bring to the table. In the Fate Franchise, there are multiple applications of Kirito being servant that can be used, and in DxD, there are arguably even more applications with Kirito as a devil, a brave saint, or even just as a human. He could even be someone associated with the Shinto factions of Yokai, or someone associated with Amaterasu herself. Kirito only really has a couple works by the same author that stick out to me Fate-wise, and DxD-wise it also never really seems to go anywhere. As such, I would really appreciate if someone wrote a fan fiction focusing on one of the two, or maybe even a mixed universe of both.