Try_hard
I believe that you should choose other members of the harem yourself, so that it would be convenient for you to prescribe a romantic line for them, and not make a Pokemon catcher out of it. But even if you choose one of the suggested options, some girls cannot be added without others. If you add Kuroka, then you need to add Koneko, because given their future good relationship, Koneko will pay more attention to her sister's man, since he wants to be closer to her. If there is a Sona in the harem, then there must be a Serafall, since she is a siskon Akeno is a complex character, as she is related to Rias, but also not related to anyone. I also suggest considering Ingvild Leviathan. (Translator, English is not my native language)
I hope you take rosewallse into account, since she is not married (she is just like al "rias gremory"), I don't know where you get that information xD (I have read the novel until vol 19), where issei fights with the one who is supposed to marry rosewallse (where she admits she would rather see rosewallse married to the man she loves xD). *and as I keep reminding many authors, you are writing fanfic, not following everything just like the novel xD.