Currently your max range of travel is measured from the colonies and starbases you construct. Without life support modules you don't get to travel very far from home. What additional purpose would your suggestion serve? Since a mechanic already addresses travel limitations, I don't see what would be gained.
There's a different thread running on the subject of borders, so there seems some interest in something different.
https://forums.galciv3.com/467659/page/1/#3567462
Influence works fine for what it represents. I have had a couple of planets I settled (one captured) that some distance away, but after a bit the expanding borders joined up. The inability to physically control areas of space that others will not just cross and build in is a bit annoying. You should look at some of the colonial expansion patterns here on earth across history. Isolated Greek colonies were annexed by Persia, and eventually became Greek again. In the New World colonies were scattered along the coastline and on the shores of rivers with large expanses of wilderness (empty space) between them. So, not stupid, just an obstacle to overcome.
