Well, I'm glad you asked! Now remember everyone, none of this is absolutely confirmed, but it's keen detective work nonetheless.
Now, if you'll recall, Reggie did say that Joker would be emblematic of the DLC. Now, all joking of PR aside, Joker from Persona 5, Dragon Quest and Phoenix Wright Ace Attorney, are all beginning to share noticeable similarities regarding Smash and Sakurai.
Let's start off with this point:
All 3 series had something going on 2015-2016, during which the DLC. Persona 5 making the splash it was with its release in 2016, DQ11 in notable development at the time to the point where they'd confirmed that Switch (at the time known as the NX) would get the game, and Ace Attorney had Spirit of Justice during that time, as well as DGS.
Now, alone that seems like a coincidence, but let's factor this in: In the same year, Sakurai had a column for each of these series, heck some of the specific games as well! He praised Persona 5, he gave a column to Dragon Quest, and even Ace Attorney has been confirmed to have Sakurai as a fan.
So, we know he's a big fan of each of these series. And let's bring up that tweet Sakurai made about the procedure for this DLC:
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That's right. While Nintendo choose the original list, Sakurai is still the one making the selections. Series that he's a fan of, have that keen boost. Of course, that's not the evidence all on its own.
Some may think, "But isn't this too obvious?". Not to the typical Smash goer. See, alot of what these guys have in relation is talked about way back in 2016, which not everyone except nuts like us may pay mind to, and even then, it's only after Joker's reveal that more zany picks like Phoenix seem this plausible!
Now, let's zero in some more on Sakurai's actions during 2016 when negotiations for DLC was underway. IN favor of the Erdrick's rising chances, Sakurai on record, had gone to visit the offices of Dragon Quest in 2016. In fact,
Sakurai was there at the interview where Yuji Horii confirmed the Switch would be getting DQ11. And for Erdrick himself, well,
Dragon Quest 3 is Sakurai's favorite DQ game:
And well, needless to say, Saks has been over there a few times. He's got a nice budding relationship with the DQ team apparently. Something similar to what was recently revealed when he visited Atlus buildings for an interview not too long ago from now.
Now, in relation to Ace Attorney? Well......
As you probably know, Sakurai has been confirmed to have met with Takumi during that same year. Now, whether or not Takumi would know if Phoenix is in, given Takumi's currently unknown position in current Ace Attorney productions, Sakurai would likely want to meet the original creator and get some tips or whatnot. Now, if they were regarded as usual pals, this photo would probably not mean as much, but they definitely don't awfully mingle a lot. Hence why this photo has been getting a lot of attention this year regarding Smash.
Case in point though. as the Judge said in Turnabout Goodbyes:
To recap, on its own, Joker's reveal merely helps Phoenix's chances, but as we've established now, there's more relatable to these franchises and their characters in general. All of them have stuff coming out this year, and if Persona 5's rumored Switch port is true, all of them will be stuff that would benefit Nintendo enough to have them on the list.
And each character is certainly big enough for the excitement factor.
Now, to bring things to a close, there are some more minor things that may be more grasping, but are things to consider.
Now, DLC is on a budget, and while I don't think it alone decides the characters, the VAs that come with them may make things easier or whatnot monetarily. Especially if per say, they're VAs already within the project.
Joker of course has Xander Mobus already on board. And with Ace Attorney.....Abby Trott is in there already. While Ace Attorney in Japan surprisingly doesn't really have consistent VAs so Abby's presence here is more notable. Again, this part is like more of a cherry than the cake of evidence, but, if they were planning to have Maya in some way alongside Phoenix, having Abby already affiliated could help them in the budget, Just some food for thought.
Not sure if I'm forgetting anything, but bottomline. If Erdrick gets in. Notice that we may have something of a pattern here, and a pretty sweet one at that.
And while Hitagi isn't the most sturdy of leakers looking right now, Phoenix Wright was suspiciously one of the few characters he didn't disconfirm when asked about.
Oh, and if all the DLC fighters are ones that HAVE NOT been represented in Smash before in any way, spirits included......welp.
Heh, sorry that took a while to construct.