The only number theory I currently take stock in is 5 new stages. Of those 5, I think 1 is Monster Hunter (probably no newcomer), 1 is Minecraft (probably no newcomer), Pokemon maybe gets one (it's the only returning series big enough to get a new stage, but it also already has 5 returning stages that weren't in Smash Wii U) and the remaining 2-3 are new franchises.
This would cement 2-3 newcomers not from a current series + Incineroar, possibly Skull Kid, and echoes. It'd match the 6-8 unrevealed characters pretty well, and I think at most 5 of those are full newcomers.