yeah...
the whole concept of interest is because the banks use what money you have with them to invest, so they're actually making money... and that doesn't happen with money just sitting in a bank.
not only that, but somebody (Duo) would need to do the Economy thing that he was talking about before... the pseudo-stockmarket thing where he has objects at a Buy/Sell rate. then, it would have to become an established pricing system throughout the game (meaning the vendors in game.)
then, the banker (Duo... staff can't "run" the investments because they can create the money... = why banks aren't run by the government...
) could use what money that the players have in the bank to buy and sell at a larger scale to make profits, then give some of the profits (interest) back to who keep their money with them.
of course it isnt THAT simple... there are lots more things that need to be done to insure stability... but it doesn't matter at the moment.
none of the above is even done yet.
of course the lottery was a good idea to help the Obsidian economy, but not thought out very well *coughs* bwahahahahah....
edit: also, the player base of Obsidian is far too small at the moment, the economy thing works when there are lots of people with lots of different things. and, there are players in game with loads of money who don't need to do/buy/sell anything... so they can mess up what little economy you could build.