Notably, the FSF recommends against[1] modifying the GPL. And, you'd want to hire a lawyer to go over whatever scheme you're thinking of.
It's easier to draft a separate license agreement if you're the sole copyright holder.
If you're not the sole rights holder, if your project contains other GPL code, you'll need permission from the authors to license under terms other than the GPL. That would include patches from other authors, unless they sign the rights over to you.
