You don't have to contact the authors to get their permission to fork the project. This is explicitly permitted by -- and, in fact, one of the primary features

You don't have to contact the authors to get their permission to fork the project. This is explicitly permitted by -- and, in fact, one of the primary features of -- the GPL. I would encourage you to contact them, though. Let them know about the changes you're planning to make and ask if they would be willing to consider merging your features in. The best possible outcome is that they are willing, which means you avoid the overhead of maintaining a separate fork! If, for whatever reason, they aren't interested in and don't want to merge your features in, you fork the project and continue on with your plan. You've really got nothing to lose! (Yes, you can rename your fork and, in fact, you absolutely should rename it.)