What if Camps/Towers/Keeps could only be upgraded in EB and your server’s home borderland? Invading servers could still capture them, but no upgrades would be possible for the invaders.
In matches where the servers’ population/coverage is nearly the same, this should have little impact, as all servers have a “safe” home borderland and are equally disadvantaged on their opponents borderlands. Eternal would not be affected at all.
However, in matches where the servers’ population/coverage isn’t the same, there would be at least one map where the server(s) with lesser population/coverage can reasonably recapture towers and keeps quickly without being zerged down due to the time it takes a small group to get through fully upgraded doors/walls. While it is possible to do this today with a skilled group, this is an important morale boost to help keep the “fairweathers” playing even when severely outmatched.
It also limits the mobility of invaders, as they would always have to run from the spawn point due to not being able to build waypoints in keeps. This allows the home server time to build up momentum against a much more numerous opponent, while making it harder for the invaders to hold on to everything.
This seems like a relatively simple way to counter large population/coverage differences without power creep or “punishing” the winning server. It simply gives the lower population servers a place to start their counter-attack.