K, some UX ideas:
1. First thing should happen, user opens the app then gets asked to provide Construct 2 location - this should be forced and checked each time, since without this the app wouldn't work correctly. If the app can detect the location, better.
2. Then... there might be conflicts with existing plugins. How would you check if the user has an old plugin version? This should be very important to ensure that the user always get the latest version.
2.a. If the user has the plugin installed AND it's the latest version, disable Download button (or provide a link to the folder since .c2addons need you to drag and drop it into C2 - terrible idea but what we going to do about it...
3. And I think that's it. Nothing else should be needed
Here's a very, very, very quick mockup of the main UI (note this is mockup, not real UI. Don't mind the colors, also). I'm torn between a ComboBox or simple buttons to select either Behavior, Effect or Plugin, but will do for now.
Words between [ ] should be replaced by an icon. ("Forum link" should have the "Forum link" text, though, same for "Homepage" (or Site link, call it whomever you want =P)
I forgot to include a scroll bar for the plugin lists, that the black square is the live preview, and if you include the update checker, there should be a button to check for updates of the individual plugin IF the user has the plugin installed.