My hot take is that I fear the days of Sakurai working on original non-Smash projects are long over, and a lot of that is because of the advent of DLC.
Like think about it, once Melee and Brawl's development were over, there wasn't anything else left to do with them because obviously the GCN and Wii didn't support DLC, so that freed Sakurai to go work on original projects like Air Ride, Meteos and KI Uprising in-between the gaps. But nowadays there isn't really much time to prioritize that anymore because now Sakurai has to work on additional DLC fighters for most of the rest of the platform's generation, as we've seen with both Smash 4 and Ultimate.
While I would love another Uprising styled project from Sakurai again, unfortunately it just makes far more business sense to have him get a Smash Bros. game out in time of the next platform's launch and then let him focus on additional DLC for the next couple of years. TBH Uprising already kinda set back Smash 4 a couple of years, we probably would have gotten it as early as 2012 if Sakurai and Iwata just decided to plan the next Smash Bros. for the next platforms shortly after Brawl was complete.