In Dolphin the user can easily define custom Service Menus by just editing a .desktop file specifying a target mime-type, a command, a name and optionally an icon.
My idea is apply the same paradigm to browsers: the KDE's plugin for browsers (Chrome/Firefox) could add custom buttons to the toolbar where each button is defined in a .desktop file. In the command field, the '%u' is the URL of the current web page. This would let the user easily define actions like "share-to-X %u" where share-to-X is a command line tool to post to the X service, like Diaspora, Mastodon etc. More details in the image below.
Tagging @broulik in case he is interested since he's working on KDE's browsers plugin.