Personally, I'd prefer that experiments be done in branches of the existing repository.
I strongly agree with that but the current infrastructure doesn't allow it for non-committers; so we have to use workarounds such as intermediary sandbox projects (e4) or to work on GitHub. It may be an important thing to reconsider for our community to encourage innovation from all parties.
By the way, technically speaking, on GitHub, all forks of a repository are actually the same repository under the hood; so all branches from all forks are actually branches of the same unique repo. Isn't it nice?