5
It would be really helpful to be able to bill clients for actual periods of time and not just when they decide to pay. For example I am not getting paid for this last month because my client is paying late and there is no clean way for me to invoice last month if the subscription simply starts whenever they pay rather than at the start.
It might also be nice to be able to send invoices to more than one email address. Or to add more than one email address to a particular client.