In order of preference....

1. A "To be sent Status" for invoices
2. Create purchase orders from Invoices
3. AutoCalc in number fields
4. Additional User defined statuses

1. A lot of companies do NOT send invoices until the works are complete but we need to diminish our stocks beforehand, as they're used, sales orders or draft invoices cannot do this. A "To be sent" status could accomplish this with ease, it won't be "Draft" then !
2. Oh yes please, we won't have to use sales orders then :-)
3. Easy to check if there is a + * - / (on blur) after the first number & if so calculate the result, nothing worse than having to bring up a calculator for such a simple task.
4. I have a custom field called "myStatus" that I update with the api, very frequently, but I'd like a nice ribbon on the right side of the doc like the existing status ribbons as they are very important & specific:-)