Thank you for all of the great replies!
Flesh them both out and make them compliment eachother. One way to ensure that one isn't outdated by the other, is to create some interdependency. I.e. to make the most out of the Side-Scroller, using the world map would give some benefit and vice versa.
This is exactly what my current mindset was as well in order to make them more meaningful as a pair. Originally I thought of if it was possible to have an "infinite sidescroller" type of thing, which terraria 2 is going to do. But then this seems almost impossible to have "macro" actions and things like wars between 2 kingdoms (as the only relationship is if they are directly next to each other or not on a side scroller map). That is why in order to have more complex macro things going on, like kingdoms going to war on the world map, npcs running trade routes, etc, I was leaning towards a world map, allowing the player to "enter" and interact with any of those macro world events happening(entering a battlefield, attacking a caravan,etc).