So actually, with all this talk I figured I would break down how this character balance has panned out over the last 15+ years. How much emphasis is put on the previous generation, and so forth. Will only be accounting for first party characters, and excluding echo fighters.
Brawl's proposal was drafted in 2005. Most additions were relevant through the Gamecube / GBA era or introduced during it, and the characters were chosen at a transitional period between generations. Lucario is the sole DS character. Note that Wario is able to play both sides here, I put him in the former since he's clearly not "new" but he does serve to represent a notable new IP from the previous GBA era.
- Recurring characters from currently active IP
- New characters / concepts from previous / current era
- Legacy
Smash 4's proposal was 2012. Lot of the same stuff going on here, and 2012 was comfortably the cutoff point between the Wii and the Wii U. Brawl's roster did not touch the Wii at all, so Smash 4 picked up the slack. The dawn of the 3DS is represented fruitfully here too. DS is done a bit dirty, it feels.
- Recurring characters from currently active IP
* - New characters / concepts from previous / current era
- Legacy
And then Ultimate's proposal in early 2016. The Switch was a year on the horizon and the Wii U / 3DS years were wrapping up, DLC allowed them to pick up the slack on a couple early Switch titles that missed the timeframe and one fresh in-the-moment add in Byleth like Corrin before them.
- Fan request
- New characters / concepts from previous era
- Post-release Switch era characters
- Piranha Plant
Coincidental or otherwise, every new Smash seems to be drafted at the tail end of a generation. And the character focus has consistently been at least 50% fixated on bringing in the fresh blood from that previous generation, to keep the roster fresh and up-to-date. So while a draft in 2021 is possible, it would be a notable formula break compared to somewhere around 2023 during a transitional period - more accurate to the series' usual flow, and roster emphasis. I honestly have a hard time crafting a Brawl-like roster of newcomers with the limited timeframe that 2021 gives us to explore.