Usually invoice generator are very strict. You input the client data, service/product amount then the it will do what is expected... generate an invoice. That's great! But wouldn't be nice to have some features besides just generate na invoice? Something like having a dashboard of all invoices generated, the amount billed. Also, if instead of everytime having to input the client information, you already have all of yours clients stored and it is just a matter of select. Also the same for the services or products, you can have them pre-stored, ready to be selected on the invoice.
Then, moving on. Something that will bring a lot of productivity, recurring invoices. If you have recurrent service, a freelancer for example. You can create a setup to automatically send the invoice to your clients, on a desired date, every month.
All the feature listed:
Invoice dashboard
Clients database
Products database
Recurring invoices