Would save a lot of hand movement going back to the top left after each feature is collected.
If you have a enhancement request, you can post it on the ideas site of Esri. There are several related to Collector:
If your request is not already listed you can add it. After creating the idea, post the link in this thread to gain more support for your idea. Esri is constantly looking at these ideas. And those that have a lot of votes may be included in a next release.
Kind regards, Xander
I think this is the link to your idea:
ArcGIS Idea - Collector - Add a Done Button to the Attribute Form
You have my vote