I think a good way to increase interaction among players of all levels is to introduce new Badges - especially for sinking ships. Since there are various Boss Ships around the worlds of the Spiral that aren't mentioned in any questions, these Boss Ships should have badges associated with defeating them a number of times.
Another rather complicated, I guess, way to increase the amount of players online we be to continue the storyline, while, if they make it too long I think I will run out of money!
They could also put more benefits in side quests because some of them just don't seem worth doing, when I skipped that level 7 side quest I never went back and did it cause I didn't see the point. Maybe add badges for a lot of those, too.
A good suggestion in general would be a search bar for badges.