Mermaid Soldier was probably the best choice to be the free revive unit for Mermaids, but they buffed Shield Soldier so Mermaids don't need a free revive unit anymore. I think the designers made a really big mistake to release Magic Doll Augment as free revive unit. They should remove the free revive give her some skill and give free revive to one of the other Augments.
My suggestion for the free revive is a unit that doesn't include the subtype Mermaid or WK. Maybe one of the deep ones (Carrier [Singer of Friendship]) or a unit related to the monster subtype (Azure Beastmaster, Harpy).