User blog comment:B214/Question and Answers/@comment-9339850-20130723025527

I wonder how Issei's children will turn out, that is depending on who the mother is. Many of the girls in the harem have expressed a desire to have his child; like Asia, Xenovia, Koneko and interestingly Irina, and im sure the other girls, Rias, Akeno, Ravel and possibly Rossweisse have the same desire as well.

The more interesting parental combinations:

Issei X Akeno... Dragon/Devil + Fallen Angel/Devil

X Ravel... Dragon/Devil + Phoenix/Pure Devil

X Koneko... Dragon/Devil + Nekomata/Devil

X Rossweisse... Dragon/Devil + Valkyrie/Devil

The one that interests me the most is with Irina, that is if Issei and her are able to actually conceive without her becoming "Fallen" and making a Dragon/Devil/Angel child.

The children that Issei could have with Rias, Asia, and Xenovia wouldn't be as interesting, as the ones mentioned above, because they would basically be like Issei as a Dragon/Devil hybrid other than that Rias' child would have some Pure Devil lineage.