After reading the Marvel Zombies series, i am thinking about a movie or a TV show that is The Walking Dead meets Fringe.
A dimensional portal opens up and the zombified inhabitants of another earth pour out into the protagonist's earth.
The portal does not last for more than a few hours but it keeps reappearing all over the protagonist's earth thus the zombies can appear anytime and anywhere. You could have a huge mass of zombies, a few zombies or no zombies at all coming out of the portal whenever it appears.
A team of protagonists is selected by the world's governments and is tasked to enter the portal when it appears and find a way to close it permanently from the other side.
During the mission to zombified earth, they find out that, in the final living years of that doomed planet, the survivors tried to create a dimensional portal to another world for the tens of thousands of survivors to find refugee in. The experiment worked partially. They could open the portal but they could not control how long the portal could last and where it appeared. The zombies eventually attacked and overwhelmed the survivors during a portal test. The portal machine was not shut down as a result.
The protagonists come across labs and government offices where the scientists and administrators have left video diaries about the progress of their experiment. The protagonists also get suspicious that they may not be the only living humans on that earth.
The protagonists have to find the testing area where the portal machine was located and find out what happened to the scientists and engineers who worked on that machine and find a way to shut down the portal machine, knowing it could be a one way trip.