Please, by all means, someone make this. I would love to see it. If all else fails maybe I’ll write it myself.
Have a portal/dimensional door to another world … aliens or alternate history evil guys pour through. Half an hour into the movie, or end of the second episode of the TV Show the heroes close the portal. Probably at great cost, maybe some sacrifice themselves or get trapped on the other side to close it. The rest of the movie/show is devoted to dealing with all of “threat” that got through before the portal closed. There could be all sorts of complications, from non-combatants who came through to bad guys that go native or join up with criminal elements.
This could make for some really interesting drama…all of the interlopers don’t actually have to be evil, but all of them are stranded here. Some want to go home, but many came here to escape terrible situations “at home”. Maybe they’ll want to naturalize, or maybe they’ll want to set up a regime here. Maybe some of them will join forces with local police forces to help hunt down criminal elements.