Oh I know why he invented her. And things are VERY fluid (he - later - had a back-up plan in case he couldn't use Amy and Rory as her parents). But he also knew that he would be taking over as showrunner, and I'm sure he was already thinking ahead. (I know that he made a great fuss about the way the space helmets worked, and how he insisted that they could go from dark to see-through, for River's entrance...)

:)