Here's a new angle I've been meaning to bring up for a while regarding speculation:
I think Jimmy is more likely to come before Rocko, Jenny and Timmy, and I say this for one good reason: dev time.
Think about Rocko's Modern Life and My Life as a Teenage Robot for a second. Rocko tends to be pretty expressive, to the point of having bizarre visuals such as those seen below:
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it's probably safe to assume that Ren & Stimpy took more dev time than most fighters right? And part of that had to do with the fact that they were very very expressive. I feel like the same logic would apply to Rocko, and thus, I feel like he'd be a character that would take a bit longer to work on
I think the same logic applies to Jenny and Timmy, but instead of in how expressive they are, it applies to their potential movesets. Jenny can essentially morph her body to do just about anything, which likely means a lot of appendages assets would need to be created for such a character, not to mention her distinct artstyle. And the same goes for Timmy I feel, only I can't imagine him working without a bunch of new assets needing to be created for his moveset. And, while I don't really like bringing this up, we don't really know the situation with Frederator/Nelvana/Whatever either, which in Jenny and Timmy's case could delay them a bit
So that leaves Jimmy. Not only did he just recently get a complete series DVD release, not only is he one of the only series repped in that online crossover series absent in NickBrawl, not only is he and Rocko the only not yet repped series in the voiceline clip files, not only would he not be as hard to literally create compared to the previous three mentioned, and not only is he debatably the most requested character for the game, but Ludosity and Gamemill almost certainly know people want Jimmy both for peoples requests for him and the large amount of requests for Hugh as well
In conclusion, I think it's a safe bet that Jimmy will be the next DLC character just based on a multitude of factors