How can we improve WHMCS?

There seems to be gap in current billing system, so to explain better see the following example:

Customer "A" is placing order on 1st of month and paying on 5th of the month as many customers do, so next renewal date should be 5th of next month not 1st.

This is very useful in many cases, one I have mentioned above and other if customer pays through Bank Transfer which can take few days.

Thanks