get a graphical gpg file verification tool into all major operating systems / desktops by default
At least gpg command line version is installed. You have to learn and use the command line or have to find and install a graphical tool, which both is difficult.
You have to install gpg, perhaps from http://www.gpg4win.org/. How do you authenticate the initial gpg download? But this website does not even have SSL.
Idea for Linux / Gnome:
Take over maintenance for seahorse-plugins. Usability (Tails example) looks very well. Get the file verification function into a good shape, get everything else into an acceptable shape (so it won't be removed from Debian) and maintain a patches welcome state for anything else.
Idea for Linux / KDE:
Contribute the kgpg. Usability (Whonix example) for file verification is not good as seahorse-plugins, but can be improved. Signature and file must be in same folder, there is no file selection dialog to choose the file. There is also no progress meter for verifying files.
Idea for Linux / Unity:
No idea, but since it's Ubuntu default there should be something installed by default as well.
Idea for Windows:
Contact microsoft and talk them into adding a gpg file verification to Windows by default.
First a tools with good usability must exist. The second step is to get them installed by default in major operating systems.