I'm very afraid that they will give us a completely useless new fairy, tbh.

Selene is only useful because A) BLMs, B) Eos is almost always redundant, and C) BLMs. Her buff to raid DPS is actually pretty tiny except when you have a party of SS-stacking BLMs. A fairy that debuffed enemies in direct contrast to how Selene buffs allies would almost certainly live on a shelf next to Eos since damage mitigation and healing are both pretty well covered without their help.

If they give us an extreme niche fairy like this, hopefully its unique abilities will be more fun than hitting a party buff every 30 seconds.