User blog comment:B214/Question and Answers/@comment-220.255.2.61-20130602172537/@comment-1359322-20130627170105

The one making the decision is the author, he's the only one that can decide whether to use Millicas' story as a sequel to DxD or College DxD or stopped the series after finish. So your preference is actually not that important. He might even go and make a prequel on how Sirzechs and Grayfia met, like he previously mentioned.