I have read this idea in passing several times throughout my study of Japanese history, and have heard others mention it as well. That is, that Nobunaga didn't want to be shogun, that his ambitions were much higher: namely he wanted to become emperor himself and start a new dynasty.
Is there any actual proof of this, or is it just more like fanfic? Given how extremely popular this period of Japanese history is, I am kind of leaning towards the latter. But then again, given how ruthless Nobunaga was, I could believe it was true. Anyone?
While this hypothesis pops up every once in a while, especially in fiction, as a possible reason for Akechi Mitsuhide turning on Nobunaga in 1582, there's absolutely zero evidence of this.
Nobunaga in fact heavily supported the emperor and his court once he entered Kyōto in 1568. This includes gifting lots of money, land, posting guards for the palace on the emperor's requests, agreeing to the emperor's request to return crown land, etc. Nobunaga also often asked the court to mediate for his many conflicts and sent letters of praise of the emperor and the court's role in diplomacy.
The closest Nobunaga got was asking the emperor, Emperor Ōgimachi, to resign. By the way it's not clear whether it was Nobunaga wanted Ōgimachi to resign but Ōgimachi wouldn't, Ōgimachi wanted to resign but Nobunaga wouldn't let him, or both (as in, two separate instances). Either way, even if Ōgimachi did resign it would just mean Go-Yōzei would've became emperor a few years early.
Also, it's not known what Nobunaga's ultimate plan was. A month before his death the court asked Nobunaga to pick between Kanpaku, Daijō-daijin, or Shōgun. He never replied before he was killed.