I believe the biggest missing aspect of all the time tracking/billing apps out there for the Mac is the ability to keep track of expenses from suppliers. Much like inserting "clients" and "client invoices" there should be an area to insert "suppliers" and "supplier invoices", along with the reporting ability of these as well (create reports by invoice dates, supplier(s), unpaid, paid, etc.).
For the "suppliers" you could include fields for:
1. Supplier ID number
2. Company Name
3. Tax ID number
5. Contact Info (Telephone, Email, etc...)
And for the "supplier invoices":
1. Invoice ID number (your own, could be set to auto-increment)
2. Supplier's Invoice ID
3. Date of invoice (date field)
4. Paid/Unpaid (checkbox)
5. Date of payment (date field)
5. Sales tax included (checkbox)
6. Tax exempt (checkbox)
7. Line items (for each include: Supplier code, Description, Quantity, Price, Discount %, Sales tax % - an editable data grid would work)
8. Category (Dropdown of editable categories i.e.; Purchases, Services, Payroll, etc..)
9. Calculations of the totals including: base price (before tax), total tax and grand total
The reporting capabilities would need to be able to produce reports by:
1. Invoice date range (from / to)
2. By supplier
3. Paid / Unpaid
The reports would need to show for each invoice:
1. Supplier Invoice number
2. Supplier Company name
3. Supplier Tax ID number
4. Invoice date
5. Invoice base price (before tax)
6. Invoice tax
7. Invoice grand total
And of course the totals at the bottom (base price total, tax total, grand total).
Having the ability to output these reports (along with your invoice reports) in spreadsheet formats (i.e.; Excel, Numbers, CSV) would be great and reduce the amount of trips I make to the accountants office.
Another thing that would be very useful is the ability to setup your own invoice numbering schemes (i.e.; "09/0001" and the next one would auto-increment to 09/0002, etc...)
Having both client invoices and supplier invoices would also allow you to create graphs showing profit/loss over time which needless to say is very helpful when you need to check on your businesses health.
Even though many of us use an accountant service to take care of our fiscal obligations and paperwork, having a native Mac app that has these abilities would help enormously in keeping track of our businesses without having to switch over to a PC to keep track of things (there seem to be plenty of PC apps that have the aforementioned functionality, but alas I have not found a Mac app that does this without being a full-blown accounting app).
Anyway, those are my two cents, hope it helps