Despite no official confirmation from Ludosity just yet, the Rocko character breakdown video hints that Nickelodeon All-Star Brawl will be receiving more DLC characters in the future and that Jenny, Hugh, and Rocko aren't the end of the game's support.
When Nickelodeon All-Star Brawl first launched, it seemed like absolutely nothing could stop it. That was, until players started dropping left, right, and centre due to the lack of voices, fighters and modes. Although that has been improved since and there was some excitement regarding the game's first three DLC characters, All-Star Brawl's player count still remains low compared to other games in the genre, leaving some worried that its most recent character, Rocko, will be its last.
That doesn't seem to be case, however. Alongside the launch of Rocko, Ludosity released a character breakdown video showcasing all of the wallaby's moves and showing off some basic combos. At the end of the video, the developer showcasing Rocko says, "And that's Rocko. Thank you so much for tuning into this character showcase. Please look forward to more like this in the future."
Although it could just be a generic sign-off message and doesn't strictly promise that more content is coming, it does seem to indicate that Rocko isn't the last fighter being added to the game and that more is on the way. Considering how much the game has been supported since launch with updates, free costumes as DLC, and with four additional characters, it isn't too surprising to learn that it's not quite the end just yet.
As for who might be joining the roster, after an inclusion like Hugh Neutron, that's really anyone's guess. Although a ton of fake voice lines were added to the game to confuse dataminers (most notably not including Hugh, who actually made it in instead of Jimmy), the original concept poster for the game might have some clues, as it contains unused characters like Wanda, Casey Jones, Angelica, Raphael, and Reggie Rocket.