bank system (essentially this with a default name) is a very popular addon for a ton of forums, i think it would work great here.
the cons of doing this though would be that you would have to add some more promotions for subscriptions to the forum
aka, premium/junior members get colored usernames, so do mods, admins etc
with a bank system you could buy lots of awesome effects for your username, everyone could.
obviously this is great but you would kill one of the top reasons for subscribing, other than ads and who really cares about those anyway?
purchasing custom titles can be applied as well.
you would also have to consider what not to add in to the bank, default banks have an item where for a very large amount of "points" (read: gil) you can change the title of ANOTHER user. this would obviously be abused.
all in all it is a great idea but would take a lot of tweaking to do, and would not be worth it if you didn't put in most of the features i listed above (there isn't that much else to do with it)