Okay, so if it is Smash related, I don't think it would be a new unique character. It's far too soon for that. But rather than some sort of bonus character, what if it had something to do with an echo pack? It would make sense for them to save something like that for after E3, as I don't see an opportunity where they could have dropped that news. Smash only really had two major appearances direct wise, and that was the reveals. Such news could also be the perfect way to bring more big reveals to a potential Smash direct.
I won't lie, this next part goes right into my wants and biases, as I have been supporting and speculating this next character a ton, but I feel like there might be a connection. I have always felt that an echo pack has been in the works after the Jane file that was with Joker began to connect to Kasumi. Sakurai almost certainly knew about P5R and Kasumi due to the grappling hook Joker has, as that becomes a major gameplay element in P5R and is something learned thanks to Kasumi herself. The Jane model showed signs of being in the later stages of completion when it was found, so scrapping it entirely on a DLC budget seems odd to me. I also doubt that it was an alt, as not only is Kasumi rather different from Joker personality wise, but she majors in what is pretty much the opposite spell type, bless. There is no way she could perform eiha canon wise, and Sakurai would know that. So, because of that, I think Kasumi was being crafted as an echo character from the very start, and they had her as an alt to save on time and money. No need for an entirely new model when you are already working on the model they will echo.
Now, is she the Starling? I am not entirely sure, but it may be possible. Could be her codename, and she does have some angelic elements to her character that could tie to the whole "messenger to the gods" thing. There was another party member who named himself after a bird(Crow), but I am not sure if that helps or hinders. Not the most confident in this part of the theory, but I guess that is why you should bring this sort of stuff up.