Not sure if this is possible due to coding issues, but why not just have one bank per account and a way to put money in the bank. This would allow trading between chars a snap. I could see that you would not want this because of twinking new chars, but if you spent the time to make the money or find the items on one char you should be able to spread the love a bit. This would especially help if you have a trade 'mule' making items for your main. I mean I hate haveing to ask people to help me with a transfer because it takes a little time to log back in. But anyway thats just my 1/2 cent on the matter and my wishing for every MMORPG to have this feature.
Thanks