It would be great to see a product from GNU about a diary entry software, a daily journal for users.
The rednotebook (
http://rednotebook.sourceforge.net/) and DavidRM Journal (
http://www.davidrm.com/) are very good examples of it. There are lot of features that can be incorporated into this new product apart from the existing ones. The product might be extended with very fabulous features like encrypting the entries with public keys. More features like cloud integration, calendar, sync, notes etc. can be included.
I don't know if this is too less or too much for GSoC. We can always add or remove things in this accordingly.