Here's my guesses for new stuff:
Characters:
Actually predicting no Inklings, just a hunch.
Characters:
- Ice Climbers (Returning Veteran)
- Wolf (Returning Veteran)
- Alph with randomized Pikmin Pluck, no white or purple Pikmin, Rock Pikmin added (Clone)
- Chorus Kids (NEW)
- Rayman (NEW)
- Dr. Mario Stage
- 3DS Stages
- Rayman Stage
- Rhythm Heaven Stage
- Smash Run added from 3DS version along with a new Co-op Smash Run
- Special Rules for 8 Player Smash
- 4th Custom Move / UI changes to allow easier custom move selection
- Balance patch
- New music from newer Nintendo game entries (Kirby Robobot, Splatoon, Zelda BOTW, etc)
- New Mii costumes like BotW Link
- Support for newer Amiibos even non-Smash ones
Actually predicting no Inklings, just a hunch.