What SCRUM does allow for though (given the stage of development) is to prototype a certain element of gameplay before completely dismissing it. For example, a story could be "As a player, I want to be able throw balls at enemies". At the end of the sprint, the feature can be evaluated for expansion, deletion or keep 'as is'.
You may want to read: http://www.gamasutra.com/view/featur...m_project_.php
As for letting go of your ideas, look at Duke Nuk 'em Forever, that is what happens when you don't let go. It must of been in development for a decade IIRC.