I mean Stargate in many ways hails from that bridge era between serial and long form stories. It has many ‘planter/ monster of the week’ type episodes, but does go broader.
If you are expecting The Expanse? Don’t. But if you have an affinity for TNG, Babylon 5, and other 90’s era sci fi? Give it a spin. I do enjoy it still.
There’s plenty of portals and planar travel in Thor 2: The Dark World. One sequence in that movie reminded me of a segment in the Animatrix where a haunted house turned out to be the Matrix glitching.
There was some sort of astral location or secret zone of human potential in Beyond the Black Rainbow, though the people that came back from there didn’t seem any happier. It was kind of like the Upside Down in Stranger Things or the terrifying world of dreams in Dreamscape. The dreamlands were just a block over from the nightmare realms of Freddy Krueger in his Nightmare on Elm Street movies, where anything could happen and they’d probably kill you.
Much nicer but harder to grok and still potentially fatal were Toontown in Who Framed Roger Rabbit and the land of the dead in Coco. (For all the praise that Chinatown gets, when I finally saw it I was miffed that the L.A. Chinatown didn’t receive the same attention that Toontown got.)
If VR or Cyberspace counts, you could stretch from The Lawnmower Man and Hackers through Ender’s Game and Ready Player One.
Edit: oh, and there was the Syfy miniseries The Lost Room, which made a brief basic cable splash in the '00s
There was also an adaptation of Alice’s Adventures In Wonderland and Through The Looking Glass in the '80s that featured a scary-as-fuck Jabberwocky and a credits list that shouted “yeah, they were available.”
Tony Hillerman novels set in the 4 corners of the Southwester US often had mystery zones, sometimes to do with kivas, ceremonial pits.
There was one where the kiva was a doorway and it turned out the Anasazi had left this world through them (the kivas were found sealed by most archaelogists when the pueblos were abandoned in real life, for reasons unknown). There was a bee gun. Don’t remember which of his books this was.
Am i misremembering? I’m 80% sure it was Hillerman, but maybe it was a Hillerman wanna be? (same setting and cover art style). It was over two decades ago since i’ve read it.