Wouldn't think new IP is something we should reject for DLC
By no means should you outright reject it, but RFT doesn't feel like a character who should be getting in what might be the final 6 slots Smash might ever have, ever... until Workaholic Sakurai comes back for Smash 6, probably.
I know there aren't many first party choices left
Wot. There's a ridiculous number, even ones people have been asking for.
- Mario universe alone you've got Waluigi as the frontrunner and Birdo as a potential second. Hell I even see people asking for Boshi as an echo to Yoshi. I'd even rather see Midbus (from Bowser's Inside Story) or Fawful (from all of the M&L games, not just BIS) over RFT.
- BWD for Kirby with others like Marx or Adeleine.
- Slippy or Peppy, anyone else from Star Wolf, or Andross for Starfox.
- Pokemon is missing a Gen 4, 5, and 8 rep.
- Zelda has 3 reps across 6 characters and could easily bring in Impa, Skull Kid, or Tingle to name a few.
- Dixie Kong has been a very popular request for the Donkey Kong universe.
- Ninten as an echo of Ness(?); Porky Minch because where the hell has he been for all of Ultimate?
- WarioWare series feels like it could use another character. People really took a liking to Ashley (& Red), but I could also see Jimmy, Mona, or Dribble & Spitz as some other potentials.
- Pikmin still has one rep and could easily bring in more as echoes.
- AC legit needs Tom Nook at some point IMO.
- Punch-Out!!, LET'S GET MIKE TYSON IN SMASH!! Joking aside, there's plenty of iconic Punch-Out!! characters.
- Xenoblade 2 was still cucked, don't forget that. Rex & Pyra definitely deserves a spot over RFT.
- Golden Sun is 1st/2nd party, right? Isaac's had quite a legacy with Smash similarly to Geno's.
That's all the ones I can immediately remember off the top of my head that are specifically first/second party.
but maybe you can at least nominate some other popular new franchises you think more recognizeable to prove that Ring Fit isn't a good pick.
Not as easy since Nintendo hasn't done many "new" franchises lately, which is generally why people don't look at them.
- Spring Man/Ribbon Girl/Twintelle (because I'm biased for her) because ARMS got kinda cucked.
- Octolings as echoes of Inklings, or hell give them different weapons from Inklings as non-echoes.
- Snipperclips? Idfk.
Honestly, if any "new" franchise should get a spot, I'd immediately give it to ARMS. Yeah it's not as popular as Splatoon and never quite took off, but I still think Spring Man/Ribbon Girl (as echoes of each other like the Belmonts maybe with slight differences, or just costumes of each other) would be a much better fit than "WFT, but different", especially for a DLC slot in what's likely the last FP. RFT would 100% bring a similar-but-different backlash like Byleth.