1. Shenmue assets, music and gameplay from Shenmue 3 to tell the remainder of the Shenmue story. There might be a tweak here or there, but you're gonna turn it on and know you're playing not much more than an expansion of Shenmue 3
This is most likely. They have the assets, the engine, and the pipeline knowledge, it would be very expensive to change this format unless they could somehow be guaranteed more money.
2. Telltale Shenmue. Decrease most interactivity and now Shenmue is essentially a visual novel with some light gameplay touches. Combat is a nonstarter.
Extremely unlikely. Telltale games are about as popular as Shenmue and are expensive to produce with an emphasis on cutscenes, writing and voice acting. It's essentially getting all the headaches of making a CGI movie with most of the headaches of making a game and very little guarantee of a big payday.
3. Change the genre entirely. Hire a competent studio to make a brawler with quality cutscenes, but with gameplay that mostly focuses mostly on combat. We're four games into the series now; shouldn't action take precedent over investigation anyway? Give Shenmue the Spikeout or Streets of Rage 4 treatment and see what happens; you'll probably pick up a few casuals along the way. I'm sure these games cost way more to develop than I could ever imagine, so maybe it's a nonstarter... Maybe the Battle Rally engine could be tweaked a bit? Sega has so many talented companies to out-source their franchises to these days....
This is more of a jarring tonal shift than anything else. Shenmue has fighting but its never really been a
brawler. Plus, this would require them to abandon a lot of the exploration systems (examining items, playing mini games, dialogue etc.) in favor of the combat system and, frankly, S3's combat system needs
massive retooling to be able to carry an entire game.
This is an interesting question, if these are my only choices I would probably take option 1 unless there was a really great dev behind the combat and cutscenes, in which case I'd take 3.
IMO Shenmue should adopt a simpler JRPG style like Persona (so less complex minigames and interactions) and a greater emphasis on combat with a well-designed combat system. Wrap this up in a compelling story that moves at the pace of S2 and I think it would have broad(er) appeal. But as I said, it'll be hard/impossible to shift gears like that at this point.