Geno never really struck me as a type that would be a DLC newcomer. I feel that a DLC spot in Ultimate would be reserved for third party characters and/or promotional stunt characters (overlap between the two can exist).
On the topic of Erdrick, while I have a feeling that while I believe that he's in based on evidence we have seen, I don't think he'll be the next reveal should an April Smash direct come to fruition. The idea of the developers continuing to use Erdrick's supposed inclusion as a smokescreen to hide the other characters that are being worked on, whether he's actually in or not, seems quite plausible in my opinion.
You can expect leakers and insiders who puts their bets on Erdrick to be caught by the impending internet fireworks show should he not be revealed, especially if the reveal is a different Square-Enix character, like 2B, for instance.
In the case of a second challenger pack reveal, it could range from a usual Smash Bros. trailer (short CGI followed by a character splash screen and gameplay footage), to a trailer with none of those (like Joker's). If I were to deal with making another trailer for a truly unexpected character like Joker, I would go with the "Joker trailer" approach. That approach makes it difficult for leakers to catch wind of, especially if development for that character hasn't fully commenced just yet. I wouldn't be surprised if the same situation with Joker's reveal happens again.