User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-98.77.243.132-20130119003120/@comment-33447191-20130121224320

Since, the author did mention somewhere, that adding new members to Issei's Harem, would cause the current heroines to lose some spotlight, and some of them, still need some heavy development of their relationship with Issei, I think that it's going to focus on the girls that are currently in Issei's harem, which is still quite a lot, for any "normal" guy to feel a little jealous - after all, at the current point Ise has got in his harem, Rias, Asia, Akeno, Koneko, Irina, Xenovia, Ravel, Rossweisse and probably Ophis as well - and each of them, will probably need a lot of screen time, to achieve their happy ending, which is what the author said, he aims for. Based upon that, I doubt that we will have any new additions to Issei's Harem, and even if they are going to be introduced they would probably have some minimal amount of screen time - something similar to one of the Sitri clan girls, which has been mentioned of having a crush on Issei. Also Kunou, or Abe-senpai, who also seemed to have fallen for the MC, and apart from that, there haven't got a lot of screen time, in the main stories plot.

Well, the author did say he would not add new harem members for a while and focus on the girls who didn't get much spotlight or development. Those include, Ravel (Volume 14), Xenovia, Irina, and Rossweisse. The 4 main girls will probaly get some spotlight as well but the focus will be more on the other 4.

As for the harem thus far, the author can introduce new girls to the harem and then add the girls to it later on. Like say for instance that Ise meets a attractive Vampire or a Magician and somehow made them fall in love with him. They don't need to be added to the harem immediately and can be added when Ise becomes a High-Class Devil where it is acceptable they can go towards his side.

- (From Animesuki)