Due to a recent "issue" regarding RM harbor i think we should see the real underdlying issue, the lack of real rewards/incentives for owning a harbor.
Also there have been cases of strong fleets (specially one in particular doing it quite often) taking a harbor one week and don't defending it at the next week because you get very few profit for owning a harbor.
Some ideas:
1-Allow to invest more than the current minimum.
2-Allow to unlock rentals with more speed.
3-Give a slight buff to rental ships, down the price for using them and allow to use them more than 3 times each time you pay the credits. Even you could bring rental SSs and CVs (just copy&paste images and fill values).
4-Allow real income for the owning fleet from repairing taxes (current sytem makes them ridiculously low)
5-Make somekind of distintive mark in the logo of the fleet owning a harbor for filling e-go.
6-Somekind of small exp buf (like 5-10%) if you are playing a ship of the national harbor your fleet owns.
With things like these fleets would have more interest in defending a conquered harbor. If someone has more ideas please share them.