Time and attendance would be a new one.
Tho, many of these apps already have integrations to Xero (et al) to do exactly this, turn time entries into invoices. So what value prop are you providing on top of that?
There are usually workflows & edge cases to optimise for. And some of these integrations don’t get the love they need to thrive.
Reach out if you want to get certified with Xero and get listed in the Xero App Store. Can help. (First).(last name)@xero.com
The main reason I started work on this was because the apps that I use did not have integrations that allowed me to do this. There's Zapier and automate.io integrations but they are trigger based. e.g create a new invoice when a new time entry is added - which is different from what TTGP does. There are also apps that do both the timesheet and invoicing and makes it trivial to import timesheets into your invoices, Harvest for example does this. But then that means you have to use the Harvest accounting software - I was not a fan of it and I believe a lot people would rather keep their existing tools vs move for this one feature.