So, do I understand you correctly that you as Yum/Dnf guys would be ok to have a different backend for GUI and non-GUI use cases as mentioned in this thread and agreed by FESCo previously? I'd be more than happy to see your sign off for this change and state this too there.
Well, it's a bit more complicated than that. Obviously there would be a "problem" with both programs offering different combinations of packages in some cases. But combined with the fact that the use case/target group of users (applications vs. packages) is different for both, I don't think it would be that much of an issue.
Wouldn't it be easier to keep gpk-application and gpk-update-viewer installed and set as default applications, and provide AppInstaller as an alternative, experimental program (also installed by default, perhaps)? In F20 conservative users could continue using gpk-* and interested users could start playing with AppInstaller. In F21 AppInstaller can be deemed stable and gpk-* can be removed from the default install.