User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20161211201608/@comment-10330822-20161213034247

From what I read, Davasena is the biological daughter of Indra and his wife Shachi, I've seen some suggestions of her being his adopted daughter, but nothing as far as who the biological father really was, let alone it being one of the Trimurti. I feel that'd be a stretch, even for this series. I doubt they'd want to make random relations to Hindu gods, that may piss off some Hindi fans who take that very seriously.

From what I've also read, there was a feminine version of Vishnu known as Mohini, that had a son called Aiyappa with Shiva, but no reference is made to them having Davasena.

Kartikeya is Shiva and Parvati's daughter, so you'd still get a Shiva-Indra combo in Kartikeya's child.

However, that sounds too fanficy, I'm pretty sure the author is fine with Issei's humble beginnings, there's no need to pull a move out of Naruto and introduce some kind of weird Jesus or reincarnation-crap. Issei's body was destroyed when he died fighting Shalba, his new body is a True Dragon's, which is probably far and away superior to whatever his relationship to Shiva or Indra could have given him, since Great Red seems to be the toppest tier dude up there with Trihexa.