I have some ideas:
What about the remaining Grimoire Heart members?
- Hades: Killed.
- Bluenote: Member of Orochi's Fin.
- Ultear: In hiding after her sacrifice.
- Rustyrose: Seeking to revive his former DG.
- Meredy: Redeemed.
- Azuma: Dead and turned into a tree.
- Kain Hikaru: Unknown.
- Zoldeo: Died.
- Zancrow: Killed.
- Yomazu & Kawazu: Unknown.
With Yomazu & Kawazu as too minor as characters, I think Kain Hikaru became a member of them. This would be a good plot twist and the final reveal, where the major members of Grimoire Heart went after the BA's disbanding.