For me, there's two main things that made me believe in the possibility of Bonus Fighters: The situation with fan favorite characters like Sora (the results of trying to get something with Disney) and Waluigi (the problem of how to make a full pack out of him without being offset from other packs) and the deadline of December 2021. And before anyone asks, I don't think Geno falls in that category. I think they would've have been too generous with the deadline of Vol. 2 if there's no plans for any other type of extra content, especially considering that it would be able to fit 3 more full packs in that time.
I think the only things that still bothers me about this idea is how will you get them and what content will it have. Bonus characters are still a lot of work and I don't expect them to come out for free. But if you pay for them, then why do we have to purchase this separate from Vol. 2 and why are they releasing it in the middle of the pass? Yeah, Mii Costumes are a separate deal from the pass, but they are mostly cosmetic, with the only exception being the Premium ones with music. It just seems like an odd way to deliver, especially considering how much Sakurai tends to think about the buyer. The way to fix this would be to make a bonus pack, but then comes the issue of how do you offer two different packs at the same time. At the end of the day, they already offer packs as a separate purchase and Plant is full priced, even though it comes with nothing else. At the same time, they offered Plant for free for a limited time, so it's anyone's guess...
On a sidenote, is there a chance that we may see a new game mode being added? I know Sakurai already said that Home Run Contest was the last one, that Vol. 2 is also the last pass planned and that improving the online would be the focus for now, but I also see this as another possibility as to why the deadline is December 2020.