I think the constraint here is that you need people to create novel objects with novel functionality in the virtual world and then sell them to have an economy. That might be tricky but if you could solve it well then, your imagination is the limit.
If people love the world they'll be happy to make things without financial recompense. Lots of folks used to run RP guilds in WoW and other games with entire worlds constructed out of whole cloth - if you build a flexible system and supply the players then DMs will emerge and create gameplay within the world - just like D&D DMs get into it for the fun alone.
