The more I look at the pedestrian underpass between Toy Story Land and Star Wars, the more it looks like it will double as a backstage access road. With the park's current design, the backstage area behind The Great Movie Ride (including service areas for ABC Commissary, SciFi Dine-In, Writer's Stop, and Studio Catering Company) is essentially an island, without direct access to other backstage areas
For the new design, it seems like this backstage area will grow, and will include service areas for additional stores and restaurants. Given this, I think they would see value in adding vehicular access, so they can restock during the day and perform other back-of-house duties without using the guest walkways
There is a similar transition in Tokyo DisneySEA, between Mysterious Island and Mermaid Lagoon (in large part because tunnels are impractical with the waterway surrounding the park), also serving the central areas of the park. From Mysterious Island, guests walk through a narrow canyon with a waterfall, then pass through a cave/tunnel and emerge seamlessly into Mermaid Lagoon. Here's an aerial photo that shows the backstage road:
And a view of the Mermaid Lagoon side. The portal itself is on the far left side of the photo, and the road is hidden behind the wall with the aqueduct-type arches:
Photos of the Mysterious Island side are tough to find, but I did find a walkthrough video showing what the transition looks like (during a blizzard, naturally). That area begins around 24:55
Obviously the architectural styling would need to change to match the respective lands in WDW, but this seems to be the concept they're using. And if it's good enough for TDS...