The Sales Order document module is a little different, dealing with expenses and what not - It is not ideal for using as a work order system. I like Zoho Books. It is nearly a start to finish product for our business. But here's an idea:
What our company, which is service oriented, needs is something similar to a dispatch or work order document module.
Currently we use the Estimate module for both estimates and work orders, but this can cause mild confusion to our customers who receive a copy of the estimate at order creation as well as at the time an actual estimate is being given.
Having a separate module for work orders would be handy, as Zoho Books is a sort of one stop shop for our business right now - which is great (less training, overhead, etc.).

Things I feel that the work order document type would need:
(1) Sales Person - I have another post about deeper sales person integration in Zoho Books, and this would fall in line with that. A Sales Person could just as easily be a Technician and that should be tied to the "staff" or some other custom type of "user" that we can add to our company settings pane. Autofill their emails, sort documents by sales person...all sorts of features it could be.
(2) Document Creation - The ability to create an invoice or estimate document directly from the work order. It would fill in all the collected data (customer, description, etc.) but then add the ability to put money into it, expiration dates (estimates), due dates (invoices), etc.
(3) Reminders - Another reminder template that would alert staff to a work order that hadn't been converted to another document. This would be a good last minute catch to ensure that technicians are performing work they are scheduled for, or a simple reminder to invoice/estimate on a job that was performed.
(4) Attachments - Zoho already has this pretty well integrated (on the web, mobile is a different story) for invoices and estimates, so similar functionality for work orders would be great (in case my customer issues a sign off sheet, PO, contract, etc.). Could add Dropbox, Google Drive, OneDrive, etc. integration as well to offload the storage from Zoho servers to cloud storage vendors as well.
(5) Scheduling - our company uses google calendar to schedule our 8 technicians in the field. It's not necessary to integrate google calendar (though that would be sweet), but we do need to note on the work order the scheduled date or dates, and the times (arrival and estimated departure).
Obviously this is not strictly accounting anymore, but Zoho has been really beefing up its Books "app" and I like the idea of being able to find all of this under one umbrella.