I might add some or maybe remove some based on the suggestions from you guys, though i don't think I will be needing to remove anyone from here.
1. Rias: The official main heroine, so why not?
2. Akeno: I love her.
3. Asia: Not much of a favorite character, but will be good to keep.
4. Koneko: No comments.
5. Xenovia: She wasn't looked into much in story, but I will try to change that and give her some relevancy.
6. Rossweisse: No reason to ignore her, especially if she comes as the teacher.. cough* cough* (Don't judge me)
7. Sona: A good character that cannot be wasted.
8. Tsubaki: Look at her and say you don't want her. You can't, right?
9. Momo: Why not?
[I will try to add everyone from Sona's peerage if you guys like, so do tell me your opinions.]
10. Serafall: Menage a twins
11. Irina: Why not make her a fallen?
12. Ravel: Look below to know the reason.
13. Lady Phenex: Oyakodon.
14. Venelana: Do I need to say anything?
15. Grayfia: No need to say anything for her.
16. Yasaka: Those mommy milkers... yum.
17. Ophis: Believe me, I am not a lolicon.
18. Kuroka: Why not?
19. Gabriel: The only female seraph... can't ignore her.
20. Le Fay: I mean... why leave her?
21. Raynare: She can be fixed.
22. Kalarwane: She too.
23. Kunou: ---
[She will be added later... very later, so don't worry.]
If there are some more, you can suggest me in comments.