This is an interesting question, and I will probably think on it to create some sort of flexible framework.

Off the top of my head perhaps something like this: for every ‘x’ amount of distance traveled across the world, the party rolls 2D6. On any double they get a worldmap encounter. You can make any kind of chart you like but I think “1=social encounter/merchant” and 2-6= increasingly difficult combat encounters” feels right.

You may privately split the world into zones so you can have some appropriate combat encounters already written up. So that a combat encounter in snowy mountains will be different than one on a tropical beach. You won’t be making up area appropriate encounters on the fly that way.

Difficultly may also scale in zones. In the area near the BBEG castle the result of a “2” will be more dangerous than elsewhere for example. You might roughly communicate this to players so guide/motivate them as needed.

Perhaps allow a perception or survival roll for them to either start with advantage in the encounter or even be able to avoid it if they wish.

Thats a very rough off the top of my head idea though.