For the Create Routes geoprocessing tool, why not add a choice to the coordinate priority drop-down to have a priority in the direction of the geometry of the input feature(s)? Often the input features may already be 'pointing' in the direction the user wants -this would also be convenient if there happened to be line features for the proposed route that increased in opposite directions, although admittedly I can't think of any real-world instance of such a back-and-forth route.
As with many things in geoprocessing, the geometry is already there, waiting to be exploited, so it makes sense that in addtion to the relative direction coordinate priorities there could be an option that pulls directly from the geometry of the input features themselves.
