Using default implementation, intefaces and Qt plugins (not designer plugins)

[tbscope]

With default plugin, do you mean the interface?
If so, you will always need to link to it.

+---------------+
| Plugin 1      |    
| Default       o---------------+
|               |               |
| Always loaded |               |
+---------------+               |     +--------------------+     +------------------+
                                +-----o Abstract interface o-----o Host application |
+---------------+               |     +--------------------+     +------------------+
| Plugin 2      |               |
|               o---------------+
| Loaded on     |    
| demand        |
+---------------+

+---------------+
| Plugin 1      |    
| Default       o---------------+
|               |               |
| Always loaded |               |
+---------------+               |     +--------------------+     +------------------+
                                +-----o Abstract interface o-----o Host application |
+---------------+               |     +--------------------+     +------------------+
| Plugin 2      |               |
|               o---------------+
| Loaded on     |    
| demand        |
+---------------+

To copy to clipboard, switch view to plain text mode 

In the schematic above, both plugin 1 and plugin 2 implement an interface (which can be abstract). All the plugins and the host application need to link to the interface.