As far as new characters are concerned, I feel as though proportionally speaking we've actually gotten a decent amount considering most of the ones in the original Splatoon held pretty specific niches. You had four (and a half?) shopkeepers, a shady guy in a corner upgrading your gear, two newscasters that pulled double duty in single player, a hero and a villain that were single player specific, and the character judging the results of your matches. Everybody has a niche, everybody has something specific they're doing.
Fast forward to Splatoon 2, and the roles of the existing characters have been shuffled around a fair bit while each new character also holds a specific role. L'il Judd changes the UI of match judging, Murch does something different with gear tweaking, and Grizzco and the Salmonids handle a new mode. It's entirely possible if not outright likely we'll see some more characters taking the roles of old characters that have moved on to fresher pursuits, but I think at this point they're probably planning on holding any more that might exist close to the vest until launch so we'll have some surprises.
Part of the issue with comparing Mario to Splatoon is that Mario is so absurdly popular and iconic that even the minor enemies have huge followings and spinoff appearances, while Splatoon doesn't have that kind of history. So you've got "hundreds of characters" in Mario, but no one would really count a lot of characters in Splatoon under that kind of quota despite being equivalents. Granted, no one's really gonna count the Octoweapons as characters either way, but the Samurai Octarian fits the same kind of niche as someone like King Bob-omb yet doesn't get the same treatment as a character. If you boiled Mario down to the characters that aren't bosses and mooks and then included "Bowser's minions" as one single kind of character, that list is gonna get a whole lot smaller.
Meanwhile Sonic's thing is that a ton of those characters are playable in one game or another, while Splatoon is played using character avatars. I wouldn't say that's an open world issue unless you start factoring in stuff like Unleashed NPCs, but they aren't really comparable anyway.
I mean, a larger number of enemies in single player with juicy backstory would be amazing, but aside from that there's not really much I can even ask out of it. Two years into Sonic you had like 5 characters, two years into Super Mario leaves you with a similar count if you're not lumping the general enemies into it.
If there's one thing I can appreciate with Splatoon 2, it's that each character has something interesting to do with the lore on top of having niches. For the shopkeepers it's more of a simple Animal Crossing-style lore of day to day life as could be expected out of the same general dev group as Animal Crossing for the same kind of character roles, but L'il Judd has a very interesting implication for existing. And considering Grizzco seems to in fact actually be run by a bear, that actually seems even MORE interesting if at all possible, assuming we ever get any kind of explanation. There may not be that many new characters so far, but each one has the potential to say a lot about the world they're set in.
Of course, those characters are so darn good that of course you gotta crave more, but looking at other video games I'd say Splatoon 2's going at a pretty good clip adding more considering the amount it had at first and the setting involved. For them to add a bunch of new characters all of a sudden the series would probably have to go dormant long enough to justify some pretty major revamps, but if your game is selling 5 million copies you're likely not going to see it drop off the face of the earth for a quarter century and then suddenly have the cast quadruple.
