At last I have been able to work the plugin system into the new editor and realize my dreams.
The editor automatically detects supported file formats and generates thumbnails for them. (Thumbnails are currently compatible with the Leadwerks system, so Leadwerks can read these thumbnail files and vice-versa.) If no support for a file format is found, the program just defaults to the whatever icon or thumbnail Windows shows.
The options dialog includes a tab where you can examine each plugin in detail. I plan to allow disabling of individual plugins, like how it works in 3ds Max.
It's completely possible that this editor could be used to mod existing games with the right set of plugins. I want to try doing this with Source games and see how easily I can load levels up. In the image below, the new editor is browsing an unmodified Leadwerks project, using a plugin to provide support for loading Leadwerks TEX files.
I wrote about some of these ideas a while ago: