My prediction: "Smash 5" will be Smash 4 Turbo. A remaster on the NX with everything in both the 3DS and Wii U versions, the DLC included (except perhaps keep Bayonetta downloadable to hold off a T rating), a few new characters (Ice Climbers and Inklings at least), and possibly other extras like new modes. Smash 4 has almost 60 characters now and the game hasn't even been out that long. Making a new game that surpasses it in content would be an an absolutely massive undertaking. Nintendo is going to want a Smash game on their new platform without waiting years into its cycle, and I seriously doubt that one of their studios is already working on it. Plus, Smash 4 could sell a lot more on a successful console. So an upgraded version makes a lot of sense.
In fact, given the rumors regarding the NX concept of a unified platform where the handheld and console can play the same games, this would be a natural next step from the two versions of Smash 4 and it wouldn't surprise me if this has been the plan all along (it could also fit with the vanilla naming of the games).
Sakurai also said in an interview recently "we don't know if there will be another Smash game."
http://www.sourcegaming.info/2015/12/13/sakurai-fe25/
I imagine if there ever is a totally new Smash game, it might be a sort of reboot with the game's core mechanics seriously shaken up and a smaller cast of characters rebuilt from scratch.