honestly, I wouldn't mind too much if DLC went exclusively the third party route from here on out
don't get me wrong, I'd love some first parties, but there's so many more possibilities with third parties in comparison
It may just be me, but there's only so many potential Nintendo candidates left before first party additions would get a little... stale.
When you open the floodgates and bring in third parties however, the sky's the limit
Never in a million years did I think we'd see the likes of Joker, Banjo, and Terry, but dang man, there's so more where that came from. Why stop now?
The best thing about third parties is that it gives a shout out to series that not everyone may be familiar with, but are still very important/relevant in the history of games.
Obviously most of us play a ton of Nintendo games, and it's only natural to want characters who you know/played as; but there are still so many important series out there that should be in. even if we're not familiar with them. Smash is a great way to learn about series that are outside the Nintendo "bubble".
I'm willing to bet a good chunk of people (not necessarily here) had their first real experience with series like Persona, Dragon Quest, Banjo-Kazooie through Smash, some of those people probably went to play the games in that series, and some of those people probably fell in love with that series.
I didn't talk about Fatal Fury because I think it deserves a special mention. As someone who goes on pretty much every big forum/board there, I really think Terry is special in how much he pushed the envelope for who could get in Smash. Since 2018 till the SNK leak, I had seen around 5 serious posts of people saying that Terry should be in Smash (one of them also mentioned Adol, which led me to make my Adol/Falcom post a week ago on page 1831).
I think Terry is a perfect example of what makes Sakurai so special, he obviously enjoys adding fan-favorites like Banjo, however, he also sees the importance of making Smash a history museum of fan-picks, first party characters, major 3rd parties, and lesser (but still influential) 3rd parties. Most people would probably just pick what the most "popular" characters are.