What if we introduced the concept of invoice in BTS 2.0?
In other words, if I owe you some money you send me an invoice via the BTS network specifying the amount you'd like to be payed and if I agree with it I can (voluntarily) send you a payment in return.
I just click on the invoice to accept it and the rest is done automatically.
This would be especially useful to keep track of small debts between friends and/or to officially request payments for services delivered.
I remember a similar feature introduced in Open Transactions
and found it very useful.
(but sadly OT never managed to release a working UI)
I've been inspired by the concept of proposed transaction in Dynamic Account Permissions
If we are going to need some kind of inbox for proposed multisig transactions in the new GUI, why not add invoices into the same inbox?