I've seen this mentioned before, and I agree that it would be the smartest thing to do.
If Nintendo and Sakurai ported Super Smash Bros Wii U/3DS to the Switch, but treated it as a new game.
By this I mean Super Smash Bros for Switch would have all the content from both th Wii U and 3DS versions on one game, but over the course of YEARS. YEARSSSSS...they'd continue to add content to the game. New characters, new stages, new music, new Mii costumes, new items, assist trophies, balance patches, and other miscellaneous patches.
Imagine it like this: Super Smash Bros for Switch is revealed to have all the content for both versions. It's in HD. 5 "new" characters available from the start. Ice Climbers, Inkling (to promote Splatoon 2, and give Splatoon a rep.), Spring Man/Ribbon Girl (to promote ARMS.), Decidueye (to promote Sun and Moon, and possibly Stars.), and Alm or Celica (Sakurai loves Fire Emblem, and there's no true limit to franchise representation. Only what each individual perceives.) I personally think Fire Emblem has enough reps, but it's also not my decision.
Sakurai is tired of making Smash Bros games. It's something he has said time and time again. If they treated the game this way it'd be a lot easier to handle. At least that's how I see it. It's taking an already full-fledged game, and adding even more to it. It's not a new game from scratch, but would still be the Super Smash Bros game for Switch.
Over the course of YEARSSSSSSSSSSS...They could add more characters to represent new IPs, new games in existing franchises, Ballot characters that received a high amount of votes, Nintendo's past, or really just whoever Sakurai felt like adding in.
Xenoblade Chronicles 2- Add Main Protag + New stage
If Ever Oasis does well - Add Tethu + New stage
New Metroid - Ridley (I can't see it, but who knows. I know he is highly requested.)
King K. Rool - for all the votes he probably got in the Smash Ballot.
Kirby and Kid Icarus are Sakurai's babies if they got new games on the Switch/3DS I could see Waddle Dee or Galacta Knight and Medusa or Hades.
These are just a few examples I could see coming to light.
If done over the course of YEARSSSS... I could see anywhere between 7-10 characters added, not included the aforementioned 5 available from the beginning.
Overall, I could/would like to see the following:
1. Ice Climbers
2. Inklings
3. Spring Man/ Ribbon Girl (alts)
4. Decidueye
5. Alm or Celica
*6. King K. Rool
*7. Hades or Medusa
*8. Paper Mario
*9. Tethu
*10. Xenoblade 2 Protag
*11. Takamaru (phased out of Assist Trophies, and turned into character.)
*12. Isabelle (due to Japanese popularity, and continued appearances in Animal Crossing. Again, phased out of Assist Trophies.)
*13. Galacta Knight
*14. New IP/ Third Party (TBA)
*15. New IP/ Third Party (TBA)
Though 3rd Party is unlikely, it isn't impossible. Dragon Quest, Monster Hunter, Bomberman, and if Kingdom Hearts III gets announced for the Switch, are the most likely candidates for representation. Personally, I'd love to Sora, but I know it's like a 0.0001% chance.
So, that's my prediction. A port of an existing game, made to it's own glory on a new system. My prediction is excessive, and full of wishful thinking. I think it fits the mark of a Smash prediction/rumor perfectly.
Let me know what you guys think, and who/what you'd like to see.