I don't understand what you mean about it being a paid feature. If someone sends a pull request to you, you do not need to accept it, or merge it with your repo. I have had people submit PRs to my repos that I simply ignored. Perhaps there is some other advanced feature you're talking about? Is there a link you can point me to that describes the feature and/or the pricing structure for it?
I think the best thing about having a public repo is that other developers could expand the code, but also pull the latest changes from you so we could always keep up with your latest versions. And none of it needs to go back into your own repo unless it's something you feel meets the specific requirements and goals you have for the project.