Stranger Things is an ok theme I guess. The first season was alright. Never needed more after the first season. (Originally the show was going to be an anthology where every season would have been a different 80s homage)
I still think the best license out there for Stern is something from Nintendo. Mario, Zelda, Pokemon...these are themes that are insanely popular across multiple generations. Nintendo licenses out their properties for a million different things. Operators would be salivating for these kinds of themes.
If they can't get Back to the Future locked down, then do the next best thing. Rick and Morty. I don't think this would sell exceptionally well, but would be a solid license.
Black Panther could be an interesting choice. It would help bring a little diversity pinball themes while staying in a comfortable comic book area. That might be a good choice in another year or two when the sequel eventually comes out
Let Steve Ritchie design a High Speed 3, but release it as a Fast and the Furious theme (and release a few SLE with a custom art package as High Speed 3)