Hidden puzzles and items throughout the world, have it be a fun and semi rare challenge to make like 25k / item or something.
Also I'm in favor of rewarding players for making beautiful builds. I don't know how the rating system would work, bug it would be a nice incentive to build cool things.
Also public works projects maybe. Have a team of people pretty up areas on the server for a set rate / 25 mins of work.
These are good ideas, but not very sustainable. The hidden puzzles could only be a one time thing for everyone and would require modding.
The rewarding for good builds is good, but that's why we have a creative server now.
Public works would be hard to reward everyone. And beautifying old Ares will become questionable of grief.
I'm for the option of an auction house depending on how its done. I've seen them be very messy in chat.
With that banking idea earlier, that could easily work if interest was involved. Something like a constant 5% growth for the amount that's in there would be simple and should be very easy for players to understand.