Are you able to release datamined but unreleased eidolons from AK? If so,✔️
Pet Possibilities: Sakuya-hime, Iwanagahime, swimsuit version of Serena, and some of the holiday outfit versions of other eidolons (Christmas Alice and Lumikki, and Halloween Zashi) - All from AK
Obviously that would be a ton of work but this is just a suggestions thread, regardless, if asked to narrow it down to 2-3, I'd say Sakuya-hime ✔️, swimsuit Serena ✔️, and Halloween Zashi ✔️.