Not too sure if this has already been suggested (searched), but how about this for the AdminCP. An area where you can browse all plugins/addons and skins for XF that are stored in a repository on your server. They could then be installed through a single click. Basically, how Wordpress handles them. They have an area where develoers can add their mods, and they then come up in a search via any Wordpress ACP. Obviously there is some form of an approval process to get listed. This would surely help those who aren't wanting to constantly upload plugins/skins, plus would not 'bulk out' the admin area.