Google offers the Gmail API to developers as a way to integrate with the email service. What app makers can do with Gmail is limited to the functionality provided in the API. This then limits what we users can do.
Get excited. Google has updated the Gmail API with new endpoints for managing settings. The additions cover filters, forwarding addresses, auto-forwarding, IMAP/POP settings, send-as aliases, signatures, and vacation "out of office" auto-replies.
With the new changes, developers can retrieve and update signatures for send-as aliases, put HTML in vacation messages, configure forwarding to external addresses, and more.
The settings endpoints are available for any Gmail and Google Apps accounts, minus a few features. Modifying send-as aliases or forwarding are restricted to service accounts with domain-wide authority. Hit up the source link for more details.
Now if only Google's own Gmail app supported adding, modifying, or removing rules for filters...