In regards to the "legacy" versus "fresh face" distinction among newcomers. Every Smash Bros. game since 64 has typically had a mixture of both. I define a fresh face as someone that could not have been implemented as a character true to themselves within the prior game development's cycle (so even if Rosalina and Wii Fit Trainer both technically had their games exist prior to Brawl's release they were both still "fresh faces" for Smash 4). A "legacy" newcomer is a character that has already been established for a number of a years, and could have been a newcomer candidate for the prior game technically.
For Melee its a bit tricky. Roy and Pichu are both obviously fresh faces (FE6 would not be released until March 2002, and Pokemon Gold/Silver was released in very late 1999). However, Ganondorf, Sheik, and Young Link might both count too (both were new characters made for Ocarina of Time, which was only a few months older than Smash 64). So for Melee's newcomers only two were definitely "fresh faces" with up to five of the game's 14 newcomers. The rest (Bowser, Peach, Zelda, Ice Climbers, Mewtwo, Marth, Mr. Game & Watch, Dr. Mario, Falco) were "legacy" newcomers.
With Brawl, things started to get a lot more mixed. Lucario, Lucas, Ike, Toon Link, Zero Suit Samus, and Olimar, 1/3 of the game's newcomer (6 out of 18) were "fresh faces". Of these, only Olimar had a presence in the prior Smash Bros. game. 2/3 of the roster meanwhile were characters that existed for years prior to Melee's release.
Smash 4 is where the "fresh faces" finally overcome the legacy newcomers. Who could count as a legacy newcomer this? Mega Man, Pac-Man, Ryu, Cloud, Duck Hunt, Villager, Little Mac, and Bowser Jr. Notice two things. One is that Palutena was not included despite her first game appearance being in 1987 (this is due to the fact that she more or less established herself as her own character in Uprising, as opposed to just a generic instrutor/damsel in distress), and secondly that of the eight characters, only four are actually Nintendo owned (indeed one of the complaints about Smash 4's roster was how many of the people who had waited since Brawl's release for some of the characters that failed to make it in, only Bowser Jr., Little Mac, and Villager had notably sized fanbases during the pre-Brawl era). Eight out of 21 newcomers (yes I am counting the three Mii Fighters separately). Things were very lopsided for "fresh faces" with this installment, and was the odd man out compared to prior game's in the series.
Now what about Ultimate so far? So far the only fresh face added in out of its eight newcomers is the Inkling (which happened to be the first newcomer announced in general). What is even more curious about the game's "legacy" newcomers is how a few of them did not even make a new game appearance this decade (K. Rool, Dark Samus, Richter Belmont), which is a trait normally only displayed by a Smash title's retro newcomer. Ultimate seems to be tilting into a Melee-like ratio so far for "fresh faces". This might be due to an assortment of factors such as the short amount of time between Smash 4 and Ultimate (only four years), the utilization of the ballot (of which would mostly be dominated by "legacy characters"), and possibly the fact that those four years were dominated by the sales anemic Wii U and a declining 3DS (we still will probably see someone from Pokemon Sun/Moon though).
Of the other viable fresh faces (ones that seem popular enough), two of them came into existence too late to probably be added for the base roster (ARMS character and Xenoblade 2). As it stands the only other "fresh faces" that could still be added in for the base roster (aside from the near mandatory Pokemon newcomer slot) are Elma, Octoling, a Pokemon from Sun/Moon as an echo, and possibly someone else from a recent Fire Emblem as a an echo.
A lot of the roster choices so far seem like they could please someone who was disappointed with Brawl's roster back in 2008 (Ridley, K. Rool, Dark Samus, and the Belmonts would hype a lot of pre-Brawl people, and Daisy possibly as well), with only Chrom and Inkling being unknowns to them. Put that in comparison to Smash 4 where the only big pre-Brawl names that got added in (aside from possibly the third-party newcomers with the exception of Bayonetta) were Bowser Jr., Villager, and Little Mac.
There is more to be said on the "fresh face" versus legacy newcomer debated, but this is some background pretext to be thought about for now.