Or maybe, just maybe...hear me out on this one guys:
Nintendo has more to reveal and release than just Smash.
Crazy, I know, but I think it's possible.
And because of this is that I believe 6 is probably around the corner and Vol. 2 will probably be done by E3 2021. I mean, think about it:
1. Smash has been "stealing the attention" ever since the March Direct in 2018, with the February 2019 Direct being the only exception (and that had no reveal because Joker was known). When the reveal isn't the headline, it's always the second thing brought up about the Direct over everything else. March 2018, E3 2018, September 2018, TGA 2018, E3 2019, September 2019, people disappointed with no Smash Reveal at TGA 2019, even with Bravely Default 2 and No More Heroes 3 appearing in the show. This trend will continue in 2020, but Nintendo knows how much Smash takes the spotlight (I mean, that's why Byleth is here as well). That's why...
2. Unless they specify beforehand, Smash Ultimate will always appear in a General Direct until it's all said and done, with a character reminder, Sakurai presents after or a new reveal. We don't know who is next, so it's new reveal time. Remember how people said we had to have a General Direct in January? This shows that people always expect Smash in a Direct, so no reveal would get backlash no matter what (unless they disclose no presence).
3. Sakurai and his team released 5 character packs in a span of 12 months since launch. Why would they take 23 months to release 6? The DLC team is already smaller from base, so they wouldn't reduce it even more, increase the number of packs and spend more money because of a longer cycle if they wanted to be cost efficient.
12/5 = 2.4 months/pack
January 2020 => 2.4 months * 6 packs = 14.4 months => March/April 2021
In terms of when the next General Direct will happen: 90% betting on this week, 10% the next one; After those weeks is PS5 reveal territory: they need to call attention to their 2020 before Sony does, because we know almost nothing so far.
As for 6 is, I'm guessing Geno or a popular fan favorite from outside the Smash fanbase (no idea who).