I don't think that is happening again just due to the fact that there is evidence that Piranha Plant was probably planned for the base game. Evidence being that Piranha Plant is the only DLC character that has or will have Palutena's Guidance codex to come along with them and the fact he was the only bonus character that was separate from the Season Pass at the moment.
There are reasons to believe Piranha was going to be in the base game- what you stated, the Spirit upgrade to Petey NOT being Piranha.
But its still a theory. We may not get another surprise/bonus character for DLC packs, but frankly, fighters passes are on a bases of diminishing returns- numerically, the longer they are continually released, the less they will make. It makes better marketing sense to give us more freebies going forward (whether that be in the form of stages, echoes, or elsewise), rather than less freebies.
Or maybe we just wont get another pass.... though I hope we do.
3/4 of the Smash 4 DLC newcomers were third parties, so it's a pretty logical assumption to make. All the characters were coming with stages which also means they would likely be new franchises, and some of the practical choices for new Nintendo IPs (Golden Sun, ARMS) had their mascots as ATs.
What reason was there to think it would be first party after Joker?
First- there were 7 DLC characters for Smash 4, the '3/4' isnt an accurate portrayal. Veterans were still PAID DLC. You cannot just discount them to make your numbers sound in your favor, that's significantly skewing the statistics and gives insight into the rest of how you will pursue a form of logic.
Second- practical choice for DLC? That's still just basing the idea in your favor. Piranha Plant is anything but a practical choice. There are many other franchises who could have gotten a character (Rhythm Heaven, Balloon Fight), or even just characters we know were considered in the past (Dixie).
Third- You didnt build any case for why Joker changed the game. I can see that you wanted to lead to an argument that Third Party characters were logical from the beginning, but the reveal of Joker did nothing for this, which is what I was saying.
I'm not sure you're actually disagreeing with me here? I do think the Fighter Pass characters will all be third-party, but I don't think we're just going to stop getting first-party characters either, be it in future DLC or in the next game. I'll be very surprised if Dixie, Waluigi, and Spring Man aren't in Smash 6.
The singular way I think we are getting another Smash with MORE characters (and not a reboot) is if its a port of Ultimate with additions made and Sakurai brought in to balance and consult. There is almost no way that a new Smash after this would not be a reboot, and likely with a new director.
I can defend this if you want, but I have to get back to moving. My biggest support is moving out of the state today. He's taking my dog friend. It's a rough morning, so I'm on Smash to celebrate and distract. But reality is still here.
On the plus side, I am pretty certain that I convinced him that the management and simplicity of Animal Crossing will be a good investment. I may have turned him into a gamer.