Some people seem to be awfully confused about what a license means.

A license (GPL or anything) is a set of rules set up by the author (or owner) of something for its usage. These rules can be anything:
- pay the author 1 dollar per used copy
- only use the product for non-nuclear related activities
- redistribute any modified sourcecode openly
- etc

You can't use the product unless you agree to the license. If the license is unreasonable (asks too much money, is too "virus-like" in your opinion) then you will surely choose not to use this product.

As mentioned, the license is decided by the author(s)/owner(s). So of course it can never be changed by anyone else. The GPL certainly has no "magical" properties that allows it to change other licenses against the will of its authors/owners. However, if the author of some software voluntarily uses GPL as part of the software, then this author, by choosing to do so, has also decided to follow the GPL license which means the entire project may need to be GPL (depending on technical details such as linking etc).

If you as a third party write a plug-in using GPL code for a project X, then certainly you have no power to change the license for X. So if X is not GPL-compatible, you are breaking the GPL by writing this plug-in.

If you pretend to write the project for some useless dummy GPL project Y that only exists for the purpose of making your plug-in compatible with X, then it is obvious that you are only doing this to avoid following the license, and it's the same as if you tried to avoid paying a license fee by juggling responsibilities back and forth between different shady companies to confuse people.

So, marcik would not break the GPL by making a plug-in interface. That is perfectly fine of course. As long as he is not involved, he is not responsible for what people do with the interface, it is something that has perfectly legitimate purposes (namely the writing of non GPL plug-ins, certainly nothing strange!).

A third party writing a GPL plug-in would break the GPL. Any amount of juggling cannot change that.