I am going to dip old school for my answer for this question. When I think open ended campaign game play I think about the need for a tremendous amount of things that your players can do. Things that will affect the story but might not “Level them up”. In this case, we are needed to find a game that has several features that will keep things interesting so our players also do not feel like they are in any rush to level up.
My pick in this instance.
Better yet with the current popularity of Game of Thrones, I would imagine that many players would jump at the chance to get in on the politics of a birthright game these days.
We all have seen now the awesome fun that can be hand making alliances moving armies and negotiating alliances. A Birthright game can set the landscape of an open ended campaign for years of play.
Best of all there are active homebrew sites that have adapted the current ruleset to 5E use.