I would love to see an advanced bitcoin client that has a number of features
1) Encrypted master wallet - encrypted with a master password key separate of other passwords in the system. It is not needed to start the program but it is needed to get the wallet file in an unscrambled version.
2 ) log in. Keeps track of users and passwords and how many BTC they can access in the wallet and their privileges such as sending transaction.
3 ) cash ledger. Keeps track of how much cash has been received and paid out.
4) settable fees for exchange, by percent, minimum and flat fee
5 ) customer information. optionally all information needed to fill out a 1099 for customers receiving enough money to legally need one in the USA.
6 ) email interface send coins. Program can log into a standard email address and send and receive email. Customer sends request to give address, program sends a secret code to email address and requests customer send email back with that code and their payment receiving address. Customer comes in with cash and that code. When cash is paid, program sends bitcoins to that address
7) email interface receive coins. Customer sends email request to pay coins. Program responds with a secret code and an address to send to. Customer comes in with that code and picks up the cash.
Customers may fill out information via email in advance.
I put this together fast, I am sure others can add/improve the above. I bet a bunch of people would be willing to pay BTC for it as an open source project. I would put at least 10 btc towards this.