User blog comment:B214/Talk About What You Want! 3/@comment-24098614-20150329012653/@comment-75.71.34.254-20150401221313

Kuroka said that Humanoid Dragon Genes are valuable and it seems like the Phoenix Clan, Valhalla* and Heaven are trying to obtain offspring from Issei as well. It seems that whatever "Dragon Powers" the children gain are potentially enough to offset the demerit of being a "half". So I don't think it will be as big a negative as  if he was just a reincarnated human. In fact it must be a real positive, otherwise, no matter how much of a Sis-con he is, Sirzechs never would have allowed Rias to choose Issei nor would Ravel's clan give their only daughter to a marraige that would diminish her.

B214: Once Rias assumes the throne, it is her descendants that will inherit. If she were to die childless, then Milacas would be next. Once Rias has children, they are in line.

Of course the whole situation solves itself when Rias and Issei have a daughter that grows up to marry Milicas. I know they would be 1st cousins, but it was at one time a common practice among nobles to marry close to conserve the Estate generatuion to generation and I doubt it would be too far out there as a plot twist in the DxD universe. It also injects just a little bit of an incest sub plot. I'm sure there is some archetype this falls under.


 * Valhalla is included because I think Odin left Rossweisse there knowing the two would end up together.