User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-5480911-20140628223319/@comment-4829046-20140629030633

Try A Certain Magical Index. It is one of the longest harem LN series and the anime is nowhere near even half complete with the manga being even further back. Granted however that tones are very different between them. DxD works good as a harem, Index doesn't.

Also have you seen Familiar of Zero? It would have well gone on longer if the author hadn't sadly passed away.