Ideas must run in the same general direction, and conceptual Integrity is probably the most important aspect of any successful project. (Read: 'The Mythical Man-Month' by Fred Brooks)
But forking is not the same as straying. Straying is letting your idea become something else -- design by (unwelcome) committee. Forking is sketching the wireframe, colouring in 10%, and being open to letting something else colour in some (or all) of the rest.
I'll leave you to colour in the rest.