I've been trying to rank up with the empire lately and thought of the same thing.
Would be even better if the missions that appear in your inbox would send you to a military station where you can have a choice in rank up missions.
Supply, data, mining, salvage or combat missions all tied into some kind of war effort narrative and have the same effort vs reward ratio.
With missions getting harder as your rank rises.
But just to start off, a simple inbox message like, "The Imperial/Federal navy has taken notice of your continued support for the Empire/Federation. Travel to station "x" to join/assist the navy in it's efforts to protect the Empire/Federation." would be a lot better than the current situation and shouldn't be too hard to implement.