I've just lost a 10hr game because my other playthrough overwrote all my auto and quick saves from my other character. It was not clear from the load game screen that it was a 15 save limit across all characters.
My suggestion would be to change the auto save limit to be per character and not in total.
Attach an internal ID to each new playthrough and append that to the file name eg. 123_QuickSave_1, 123_QuickSave_2
