I think it would be nice if you could add the ability to remove the plugin files after I click 'Uninstall'. Is this possible for the 1.6 series?
That's not possible because the system doesn't know which files belong to the plugin.
No - that is up to the author of the plugin. It's practically impossbile for MyBB to detect what files the plugin adds to its file system.
If there was a plugin uploader it'd be possible, I have this feature in this plugin:
http://mods.mybb.com/view/plugin-uploader
Just stores a list of the files when then plugin is imported and can then remove them when you want to get rid of the plugin.
Nice feature! I would like to have a plugin uploader built into MyBB!