The offered default is the current branch, therefore there already is a key binding for creating a new branch starting at the current branch. But that binding is b c RET, not b c.
Okay that was a slight simplification. If and only if there is a branch at point (e.g. in the references buffer) then that branch is offered as the default instead.
By the way some changes are planned to how branching and pushing work. But that will take some time.