Guys, I agree that the Ethereal part is not the best right now. It's functional, but generic, and I am aware of this. I simply haven't made anything better yet, but I hope to do so eventually.
The most straightforward solution would be some sort of alternative acquisition path. In X-Piratez, for example, you can buy faction leaders, although that is too straightforward.
Specifically for "Important Ethereal" I envision something like "Important Ethereal Ship Luring Beacon" that a player can eventually craft (using tons of resources, maybe including live Ethereals), then research it, then the percentage of "Important Ethereal Ship" mission appearance chance per month becomes 100%.
Something similar (manufacturing + research) should be done with Mib Leaders. May be some sort of multipart disinformation champagne to eventually trigger Mib base construction.