There are a few relatively simple ways to create more long(ish) term money sinks.
1. Making classes/levels relatively close in terms of performance, but with larger differences in cost.
For example:
Mk1 cost 2 mil armor x 2.3
Mk2 cost 6 mil armor x 2.4
Mk3 cost 15 mil armor x 2.5
That gives noobs a chance to upgrade earlier while keeping something higher to work towards, without making the weaker option totally not worthwhile. Classes that follow a similar philosophy can also be created for shields and buyable power cores.
2. Remove cargo/flhook restrictions that keep / discourage people from buying a lot of gear on a ship. For example: no cargo space taken by armor, shields, guns, since there seem to be better ways to keep people from mounting CAUs on snubs now. Make ships more upgrade-able.
3. Instead of character whipes, make some gear deteriorate or downgrade over time. For example, turn all CAU8s into CAU7s once per year. In order to avoid people having their CAU8 downgraded shortly after buying it, turn CAU8 into a non buyable CAU8 first and then the non buyable CAU8 into CAU7 a year later. (if power core or shield classes are added, same for them)
4. Divide all prices of gear, ships, and commodities by 5 or 10, so character corruption wont become and issue before you pimped out your ship to the maximum.
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But before we even talk about adding money sinks, there is something that needs to be addressed:
The way things are now, more money sinks / grind / trade doesn't actually raise player interactions. The routes are too spread out, piracy is a non-issue for traders, piracy is too boring to be widely practices, so law enforcement has nothing to do. As long as the problem of too many possible routes remains, no amount of necessary grind to compensate for money sinks will have any positive impact at all, and it will just make the game more boring instead of more active. The number of possible routes and jump connections need to be reduced drastically and without delay, after that you can look at the economy. Trying to reform the economy before making jump connections more conductive to random encounters is pointless and counterproductive.