At this point I believe that Issei is going to end up married to:
- Rias (Issei will kiss the bride while looking Raiser dead in the eye, haha /bad_joke)
- Asia
- Akeno
- Koneko (Meow!)
- Ravel (once Rias and Issei marry, this would be pushed by both families, even though it would eventually happen on its own)
- Xenovia (I also predict she will be the first to fall pregnant)
- Irina (and the wedding will be done in heaven by Michael himself)
- Rose (just basing this on some spoilers I have read)
- Ophis (she will copy the others, plus from what I read she does have feelings for Issei)
- Le-Fay (she just needs a "flag" to be set, such as Issei suggesting upgrading to a lifetime magicians contract or something)
- Bennia (The biggest Oppai-Dragon fan ever, more so than even Le-Fay, so why not?)
- Serafall (So this is just my wishful thinking, but that falls under speculation. A Maou can't have multiple partners of their own apparently, but I haven't read anything that suggests they can't marry into a harem. She is already on good terms with Issei, so just needs a "flag" of hers set also)
- Kunou (though she won't be able to live with him, unless they all move to Kyoto)
Never to marry but will end up living with:
- Kuroka (Are you thinking that Koneko will allow her to marry Issei? No way!)
- Sona (She has been spending time at Issei house recently, but she wouldn't want to marry because of her friendship with Rias)
Anyone I haven't listed I haven't yet read enough to form an opinion.