Once you have earned a lot of money with trading/mining or gathered enough loot with pirating / assassinations you need something to spend you money on....
How about buying your own starbase, or outpost ....
Ideally, you should also be able to decide the location of your new real estate to choose a neighbourhood of your liking.
It would be even better if you could purchase certain modules to customize your base.
For instance buy your own police force to protect you when you're coming in hot with pirates, police on your tail.
Extra storage space.
Advanced radar system.
Perhaps a cloaking system to hide from the law.
Some nice turrets to play with, including a target practice system (think clay pigeon shooting, but then with meteroids, or your enemies, ...)
I think the issue is that once you're up to the point where you can buy your own starbase, the rest of the game becomes pointless anyway because of the amount of money accumulated :) It'd need some radical redesign and expansion of existing game features, especially missions targeting players with more money / better elite status.
Well you have a point, but if you look at games like Oblivion and Skyrim, part of the fun of gathering a lot of money is to have some decadent things to spend it on.
One of the design guidelines for Pioneer is that the player is just a small, lonely character against the background of a vast universe. While you should be able to influence matters to a degree, the rest of the universe will continue just fine without you in it. For that reason things like colonising planets, rising to the top of mercenary organisations or owning/controlling captial ships or starports are not something that you're ever likely to see in the game.