I have some ideas to extend the forum with some funcionalities as export messages to PDF (it's just an example).
Is it possible to achieve something like this through a mod_forum subplugin? That would be ideal, but I'm not sure if possible.
Another way would be to create a local plugin and call a AMD module at "lib.php" to create a button and insert the button inside the forum as well as all the funcionality, but this sounds more like a hack.
The last option I can think of is to include this funcionality through the theme, but this approach complicates installation in different environments. It also breaks with modularity.