The gaming group that I belong to tends to not do open ended campaigns. Or maybe the undecided plan was to go go go until GM burnout, open ended until then. The Pathfinder campaign I ran was based on an Adventure Path, which had a built in ending. The last game I was in was a Mutants and Masterminds game that had an ending written in by the GM. A clear objective and then roll credits. So I can't say I've been in many decidedly open-ended campaigns.
If I were to run one, I believe I would consider what system would dictate that type of game. I would want a lighter, less complicated system to run in the background while focusing on the story.
I found the lighter game systems would be best for me for open-ended campaign play. There's a collection of games that are similar in crunch and have similar mechanics. Those games would be Savage Worlds, Ubiquity, the Vortex system, and Fate.
These systems seem to often be compared to each other. They are all less granular and crunchy than systems like Pathfinder or GURPS or Hero. But they are also not considered light systems. I've seen them most often described as medium crunch systems.
One game mechanic that is almost universal across all of these is the usage of tokens or chips or something that the player collects and spends. These games have a sort of economy fueling events and story in the narrative. Savage Worlds uses Bennies. Ubiquity calls them Style Points. Vortex named them Story Points. Fate Core and Fate Accelerated have Fate Points.
Not all of these games use them exactly the same. Some flow more than others through the course of a session while others reset. There are other games that incorporate similar mechanics and the more crunchy games mentioned above even have optional rules that mirror this kind of stuff. But the examples I've listed kind of require it. They really wouldn't be the games they are without this mechanic.
I feel games with this level of detail and have this story altering mechanics lend themselves the best towards open-ended campaign play games.