Eh, I think the main difference with Piranha Plant is that he was the, for a limited time, free character. It'd be a lot harder to sell Piranha Plant in a Challenger Pack for $5.99, as opposed to Joker or Banjo-Kazooie, especially alongside Packs filled with content entirely new to Smash.
I personally think there are some great First party fighters, and I certainly wouldn't be disappointed. However, I think the much more appealing prospect (for me, and potentially simply the concept of the game going forward) is continuing the crossover with more 3rd parties. I do think that while some of these first party candidates would be fun, it all comes down to the fact that they might not be considered in the first place, but also that precedent isn't on the side of popular side characters from 1st party franchises when it comes to Smash DLC.
Not really going to go deep into it, because it's not really an argument, but something that neither side will budge on. it just comes down to the vision of the game. I personally think they'll continue down the road of 3rd parties, because it's provided the most successful digital sales they've pretty much had ever. But, I could be wrong.
It'll probably just come down to that 6th fighter to set the tone. If you ask me, they'd want folks to know 1st parties are in contention in the next pass (then again, what we think is what the minority thinks, basically) and would therefore start with a 1st party. But if they start with another 3rd party I have a feeling they'd stick with more 3rd parties going forward, and I think if the "record" is a big deal for Sakurai to expand, they'll add as many new series to the game to achieve it.