Lord_Shiva_
Well the three harem members are not bad but I would like to make few suggestions since Sirzechs is so powerful and devils are lustful beings some more powerful women by his side would be good. Tiamat - Chaos Karma Dragon (DxD) because it will be a better way to not only get a strong waifu but she could reign in the dragons under her with help from the MC. Serafall Leviathan (DxD) - She is one the strongest female in DxD and not to mention as a fellow maou of the underworld would be quite helpful to the MC. Yasaka (DxD) - A very powerful youkai who rules over kyoto and has good relationship with the shinto pantheon. Kuroka (DxD) - She has a lot of potential which got shelved because of the experiments done on her so she can be a good peerage member for the MC. Poison Ivy (DC) - She is very powerful and highly underutilized, not to mention that she is a avatar of green as well. Mera (DC) - She an atlantean and quite powerful in her own right. Not to mention that she can bring Atlantis as a ally to the MC. Powergirl (DC) - A kryptonian whose potential is just limitless. Also not having a kryptonian waifu in DC is a crime in and itself. Talia al Ghul (DC) - This one is because I personally like her charater and she feels quite wasted in Batman and can be trained to a much higher level with the help of magic and some super soldier serum. Blackfire (DC) - A royal princess through which the MC can get an entire planet of Tamaran under him. Not to mention that she is very powerful in her own right.