User blog comment:B214/Volume 22 spoilers/@comment-202.67.40.227-20161030103910/@comment-10330822-20161103003836

There are few Ultimate-Class Devils. For example, Ruval is supposed to be way up there in the Rating Games, and he is the heir of the Phenex Clan, yet he is only now be considered for an Ultimate-Class promotion.

Issei is marrying Rias, so he's becoming a Gremory either way. What's more, he's going to be a Duke, one the highest ranks. Even if he was given a title, it would automatically be absorbed into Gremory after he married Rias and became the Duke. Unless Issei can become a Great King or something, he's not really going to have his own House. Of course, Issei's children in Ex, some have Hyoudou as their last name. This implies that the name exists as sort of a lower house after his marriage to Rias.