There's nothing tangible to point to (unless you spin things like "Shulk isn't a character who'd usually appear"), but comparing the newcomer casts of Brawl and Smash 4, the focus seems to have shifted from filling in the missing characters while still adding unique movesets to selecting new gimmicks first, then pairing them with characters. Which, of course, you have to evolve the selection from title to title to stave off staleness, but I don't think we've reached the phase yet where it can be said that "we don't have anyone big left to add so let's just double down on gameplay gameplay gameplay, who cares who's in the game anymore."
Gameplay is essentially a fancy formal outfit that each character is wearing to the fashion show that is Smash. Naturally, you want every character to have the best, most personalized outfit possible, and definitely not to force anyone into showing up in rags, but as the old saying goes, the clothes don't make the man (and can in fact be changed from show to show, like gameplay).