The consensus in Heroes is that healers are pretty bad. Full disclaimer that these changes will probably never happen, since gacha apparently cannot make big changes to game mechanics for reasons, but it can be fun to speculate all the same.
Healers are borderline useless in Arena, and the modes where they actually pull their weight somewhat in Tempest and Chain Challenges, but a Falchion wielder with Renewal and Reciprocal Aid can do their job better and still contribute offensively.
They main drawback with Healers is dealing half damage. Wrathful Staff fixes this a bit, but is locked behind a 5* unit as well as taking up the B slot. I find Dancers are superior to Healers, and they do not get the penalty to their damage dealt. They get a slight BST penalty, but that barely makes a difference.
One fix would actually be making Wrathful Staff more accessible, by letting a 4* by more available for SI fodder. A more extreme would be more drastic, but I suggest just nixing the half damage mechanic altogether. Healers already cannot get damaging specials, as well as being restricted from a multitude of other skills. On top of that, their best (in terms of might) spell only deals 10 damage with no secondary effect. I've heard people say that not giving healers the half damage buff would let them take on low Res units, but I don't see that as a problem. Being effective against a small array of units is hardly game breaking where a meta revolves around OHKOing units seldom leaves healers with a job to do anyway.
If the damage nerf was changed, I could see healers getting a BST penalty along the lines of Dancers, which I think is what they should have done in the first place.
It will probably never happen, but pulling Lachesis and breaking my pity rate made me ponder on what could have been.