I’m by no means an expert on 3rd parties, but I definitely feel that 3rd parties should be “legacy” series and characters. It shouldn’t be “oh, that game came out last year and is super popular? ADD IT!”
The concept of third parties in Smash should be reserved, in my humble opinion, for characters that are iconic to the series they come from, have a gaming legacy (something I believe Sakurai has addressed) and popular appeal. There are many characters that I feel address all three, and those should be the characters chosen.
While I am not a fan of Steve as a Smash pick, Minecraft does bear a legacy. I still do not believe he’s in, but that’s a separate conversation to be had. Fortnite, while popular, does not have a legacy. It may have a legacy in the future, but that has to be proven.
All other 3rd parties (Save for Bayo) bear a sort of gaming legacy or a reason to call them a gaming icon. It’s not something I can prove, but I would argue that the 3rd parties we have also outshine several that are brought up online and on forums.
I would say that, as a 3rd Party, if you aren’t as recognizable or well known in the gaming community as Rayman or Joker, chances are more against you than in your favor. This obviously includes Japanese popularity. Persona and SMT may be niche in the US, but are crazy big in Japan. You need the popularity in both regions to succeed.
That isn’t necessarily provable either, but this seems to be the same thing that weave always seen. A new 3rd party gets in, so now everyone and their mother of 3rd parties are considered more likely. This happens every time. It happened for Brawl, it happened for Wii U, and it’s happening again now.
I’d temper expectations, but I also wouldn’t buy that leak going around until we get actual proof.