Just came back to pop in and see how everyone is treating the e3 DLC characters Hero and BK.
My 2 cents is still that Geno is in, for the same reasons as months ago. But it's also telling how after this e3 announcement the conversation has become, "Who is left to add to Smash Bros?" Geno is now by far the oldest-most-desired character that is not in the game. He's the last ultra-hype character.
If he's a costume then that's a total deconfirmation for this game. Costumes have served that role in the past and there's no real reason to expect differently. But I can't imagine he'd be in the fighter's pass along with Hero or BK. The Joker pass's costumes, stage, music, and spirits were all related to that IP. It's billed as Smash Bros X Persona, Smash Bros X BK, and Smash Bros X Dragon Quest, not Smash Bros X Dragon Quest And Rando Characters From Other IPs.
The one and only one thing Geno has against him is that he and Mallow are spirits and each have an adventure mode battle modeled after them. But that's still kind of a big deal, because fighter passes add stages, spirits/spirit battles, music, and costumes along with the fighter. Geno already has a couple spirits and accompanying battles, so there's less content to add. You're not just selling Geno, you're also selling Mallow/Smithy/Croco/Booster costumes, a few other spirits, some lit music, and a stage (my guess is still the Bowyer boss battle in the forest with the lock-buttons gimmick). That said, iadding him last makes the most sense, if only for the logistic reason that they'll be getting less requests--if you add Geno not as the last DLC character, everyone is going to think their currently deconfirmed spirit is on the table for subsequent DLC and flood Nintendo with requests again.
As a last nugget, here's some out-there speculation. After Geno, the rest of his fighter pass content is more or less a stretch. You could add Paper Mario content by lumping in the Paper Mario games with SMRPG as the forefather, but it's insanely unlikely, they're different IPs owned by different companies. BUT, if they bought the license for Geno and Mallow and created brand-new PAPER GENO, making him a Paper Mario IP character, that opens up the floodgates for all the Paper Mario characters for costumes, music, spirits, and stages! You've got massive hype-building and promotion infusing the announcement of a new Paper Mario game. New game, brand-new Paper Geno and Paper Mallow included!