A quick thought maybe plugins being browsed and installed via the plugin manager would give greater access to user contributed and developer contributed content.
The plugin manager doesn't install anything, it just selects which plugins are loaded into Geany and lets you set their preferences. It can see all plugins that are installed.
A plugin browser that looked over the network (say like Firefox) requires all plugins to be in the same place. That is not the case, because plugins can be provided by external sources. To try to get some consistency, a plugins package has recently been created to group some of them, but not all developers can or wish to meet the requirements or schedules for that package so their plugins are not part of the package. The Geany website points to the plugins website that tries to keep track of all the plugins we know of. All the plugins I referred to before are part of the Geany-plugins package.
Cheers Lex