Retained Hours (Balance of Time)
It's becoming increasingly common for various types of software and IT firms to have clients to purchase hours upfront rather than the traditional model of billing for services as performed. There are various benefits to both the client and business.
Many other project management tools on the market have already supported this for years, or even decades. Essentially, when a client purchases a block of time they'd have a positive balance of time. As tasks are performed the time is pulled from the present balance of time. No invoicing needs to occur unless the client runs out of time. This would be a great feature to add to the project management module.
While I understand this can sort of be circumnavigated by adding account credit to that client's account within WHMCS, this isn't ideal. Most firms like to offer a discount when hours are purchased upfront, and tracking by credit balance would not support this. Tracking by account credit for hours can also lead to some confusing bookkeeping and even the potential for legal compliance issues depending on how your country/state/province/city regulates prepaid cards and gift certificates.