Imagine if every map of Screeps would be like planet Earth but with the shape of a Square, the rooms in the limits of the map could connect with the ones in the opposite direction (limited to 1 shard).
This can connect the game map a lot reducing distances and making the game feels more realistic.
This won't limit the expansion of the game map, because adding or removing zones in the limits it will only change the distances to reach the opposite zones by walking.
To make sure you undertand what I'm suggesting, take a look at the diagram:
I did this diagram with draw.io, if you need the XML to modify the image, just request it.
This idea can be modified in any way to fit better in the game, this is just a suggestion on how to do it.
I hope I explained myself well enough, thank you.