User blog comment:B214/General Discussion/@comment-24098614-20170705203633/@comment-10330822-20170706074656

Ravel obviously isn't an heiress because she marries into the Hyoudou family in EX. That's why her daughter Robertina is Robertina Hyoudou, not Robertina Phenex. Ravel "leaves" the Phenex's to marry Issei.

You are right, Ruval is the heir, not Ravel, Ravel is the youngest, so she was never in a position to really inherit unless all three of her older brothers die. Really, it is more practicle that Ravel was to be married off to increase blood ties with other clans, since she is a rare pureblood. Similar to how Riser was going to marry into the Gremory, since he isn't inheriting his family's title either.

The real friction between Issei marrying Sona is Rias, not Ravel. Because Rias is actually an heiress like Sona. However, since Millicas inherits Gremory's heir status in EX, Rias is no longer an heiress in the future, meaning that friction is also gone.

So yes, Issei probably could take Sona as his wife if EX pays out like its supposed to, with Ravel joining Hyoudou and Rias abandoning her title as heiress. However, Issei doesn't really seem to be Sona's type. She thinks he's "wonderful" because of how he treats Rias and women in general, but I don't think she's attracted to him too. She would be more interested in an intellectual. After all, that was why she broke off her betrothed marriage, because her suitor couldn't beat her at chess.

An intellectual suitor would be a nice addition to the story. I feel Sona and Saji's relationship doesn't have much attention at all. Saji needs an opportunity to impress Sona like Issei did Rias, and a rivalry with a worthy suitor may cause that.