The bar will always be raised. It appears most fans just want new characters. After Ken's and Incineroar's addition, it seemed reactions from certain pro players hoped that wasn't just it. I'm past the roster reduction, only because if more characters are added, I just hope the current characters will be gradually tweaked to improve their properties and moveset. When Samus went from high mid-tier to low low-tier in SSBB, I was disappointed. She improved in SSB4, and I'm hoping due to the different style of play she'll at least be decent.
Sakurai and his team have really gone out of their way to spoil us. Sure, some people didn't get the characters they had hoped for, but all this content is fantastic. The new and improved training mode, the online options, the variety of stages and music . . . Even the concept of a final smash meter was added. It makes me think that perhaps Sakurai has people checking out what fans want. If that's the case, then I think he and his team can meet our expectations. I just think right now, Sakurai needs to take a break. He joked about it in yesterday's Nintendo Direct, but I think there's a bit of seriousness in his comment as well.
For now, just enjoy SSBU. The Switch is still recent, so SSBU should keep us occupied. With a development team, there's going to be all sorts of ideas to come up with. Even if we ended up with the same kind of content like we have in SSBU, I think the focus would be mainly on improving what's already present.