All developers would upload their plugins to the store, even the free ones (like in Unity store for instance). Then C3 would have a built-in addons browser with an "install" button.
This would be an amazingly valuable addition for many reasons:
1. People would be able to find addons easily (right now they are lost in the depths of the forum)
2. Addons could be automatically updated or at least notify user that there's an update available (right now it's hard to follow with addons updates)
3. All addons would be installed from trusted source (Scirra store)
4. Would be possible to restrict bundling the paid-addons (right now this is a quite serious problem)
etc.. basically it would make life much easier for everyone and would solve many issues which are seemingly unrelated, like this one https://github.com/Scirra/Construct-3-bugs/issues/903
BTW: This could be easily added to C2 as well as a kind of iFrame.