Just a few thoughts,
Regarding rules, don't you all feel that Sakurai actually has and follows his own rules for characters? There's the exclusion of non-videogame characters and that it seems Sakurai also tries to give his all in all entries. He seems to hear fans to a degree but not in a way it dictates everything of what he does in the game.
I understand the desire for upgrades but at the same time I don't see upgrades for one simple reason, do you actually all think that the pool is so small that upgrading is the only option? Nintendo whether we like it or not, releases new entries to franchises and attempts new IPs every now and then, I see them more likely to add them not for promotion but for pride in knowing they keep making new stuff. There are also popular 3rd party franchises that are quite attractive in regards of crossover potential and from companies that are close to Nintendo.
Case in point, one of the most popular upgrade talk is Waluigi but just think about him he has been passed several games and even Piranha Plant got in before him. Does this means he doesn't have a chance of being playable? Of course he can be playable but so far I see little to no incentive from Sakurai to add him and with Nintendo making new entries to their own franchises, creating new franchises altogether and introducing new characters, I just see the situation difficult for AT and spirits to some degree in general at least for this game.
Is there actually a pattern for dlc? Yes,
DLC for Ultimate seems to be aimed to different groups and fanbases as well as being from series of different impact or popularity as well different eras. And I'm going to be honest, there is not a single character that will appease every group.
That said, the theory of Hayabusa and Byleth being changed? That sounds incredibly silly if only because I don't think any of the two characters can carry being the starter as well as Joker. Which I would say a character like Sora or 2B would be more fit if its a single reveal due to their popularity worldwide.