I kinda mentioned this earlier, but I don't think Sakurai would give a "Total number of stages" list if they were adding more in the base game.
After referencing the "103 stages. We must be crazy", he mentioned music. He said, "This total is also Ultimate." He then listed over 800, and then over 900.
To me, this implies that characters added from this point forward wouldn't require a stage. This brings characters like a Pokemon, Isabelle, Shadow, Dixie Kong, Ken and even Geno to mind. Granted, I think we're obviously getting DLC for this game. In DLC, I think Spring Man and Rex & Pyra are practically guarantees here. Both could provide new stages from their respective games. Regardless, there are 5 slots left for stages on that menu, and there's definitely the chance they could be added later, and weren't added now because they'd spoil the rest of the roster.
Either way, there's a reason those blank spots are there.
A Sora & Roxas newcomer and Echo combo would be pretty badass, and would give us a stage like Twilight Town. Not to say I think it's a likely scenario but I think it'd be cool.
EDIT: Another point I wanted to make despite it seeming as if most people have caught on. Echo fighters seems to be based on a character that comes from the same series. I don't see it often here, but there are plenty of people on other sites suggesting clones from completely different series.
If you ask me, Echo Fighters will be characters from the same series of their base series. So basically, no Jody Summer based on ZSS, or some other combination.
I thought this would be obvious, but apparently not. That isn't to say it COULDN'T happen, but I don't really see why it would at this point.