There would be a Basic : public BasicIface implementing the "default" behaviour and passed to the factory function of your real plugin infrastructure. I hope you get what I mean. If not then say so and I'll explain more.
I am not sure, let me think about it a bit, and I'll ask if not
The pure virtual interface was part of the design in my last post.
This has to be the case anyway, since Qt plugins demand (fully) pure virtual interfaces.

It (what you are suggesting) might however be similar to an improvement I was (still) thinking about to my last post:
What I didn't like in my last post is the fact that the logic (the default behavior) was moved or mixed with the application, which is basically the UI.
So what I think to do now is as follows, and it might be similar to what you are saying:
I will move my default logic to a regular DLL, and have that DLL, work with plugins.
All the plugins, as said, will inherit the same interface.
I think this is the best solution I came up with for this problem so far - but if you think you have a better, do tell!