It might not be such a bad idea, maybe 1% per week? However, you should not be able to leave your account for a year and come back to find a huge amount of gold.
Saw it today, and it was pretty dam good. Nice graphics etc. Another thing that I noticed was that all the miltary vehicles could still go when all the other ones were screwed up. Good overall, despite a few flaws.
For the people who are saying it would slow down mining etc, the bank would be SEPARATE and for rares only. How can the bank account be shared, they mean only you can acces your own one which needs an addition password. Great idea, but Jagex will never do it.