Reoccurring invoice are great but lack alot of necessary things such as pro-rating and the ability to change other prices based on thing being billed. Here is my ideas:
Have an option to turn on Service Billing
on the customers page you will have a place to add services that are billed on a monthly billing date. The services will be a inventory service or item plus a set of rules much like a contract. If say we have a customer that goes with our MSP package that does not include onsite service but does include remote service then when a tech bills them for remote service the pricing for the work is based on the service on the account. then in the middle of the month they want to upgrade the service the next bill will pro-rate and any work will reflect the changes in the prices as well.
This would also be helpfull when billing things like Office365 when a customer adds/removes licenses.

I would even be willing to pay more for a feature like this. perhaps as part of the MSP package.