I think I get it now.
The way I see it, you'll be able to navigate as usual, and if file happens to have a "related" file, like .design files for C# or VB, or header for C++ source, then an extra button will become available. You'll be able to either press that button to open a "related" file, or use whatever new keyboard hotkey I'll come up with for that.
I've added your request to the queue. I'll try getting it into the next major release after the next one, i.e. into v2.10.x as oppose to v2.9.x one. Thanks for your suggestions.