It would be extremely helpful to be able to manage basic accounting in WHMCS, and the Transactions feature comes close, but is not optimized to manage expenses very well. Please consider adding support for recurring expenses, vendors, and expense categories to the Transactions feature. Only these very basic additions would go very far toward allowing the minimal accounting features needed for many users, and would likely be helpful for add-on integrations with accounting software as well.
It would also be helpful to have various normal reports for expenses, such as total expenses and expenses by category, and if possible, projected net income that could anticipate income in future months based on recurring expenses.
If you're feeling generous, also adding the ability to attach a receipt image to expenses would be amazing, especially if we could use the mobile app to snap a photo and attach it.