I was wondering if it would be possible to make every unit able to reclass via heart seal into every class in Fire Emblem Fates?
Unfortunately, probably not.
I think it’s possible to force promote any class to almost any other class (prob not like, demon lord, tho) through events, but I don’t think you can tie that to an item, nor allow the player a choice of promotion.
I’m pretty sure there are patches to allow for 3 promo options instead of 2, but more than that is hard to do, and any class might be impossible.
Edit: Just crossed my mind that you might be talking about Fates, not FEGBA. If that’s the case, I’ve no idea.