In light of the addition of Drowned's in 1.13's Update Aquatic, have you ever considered the addition of Depth Hags? Sure, Swamp Hags are technically already water bloated and swampy, but, I thought a variant of them that hides below the mud patches in swamp water, and that swim up out of the mud (similar to the spawning of a dreadful peat mummy) to attack would be an interesting idea.

They would likely look slightly more algae and mud covered/more skeleton (given they would have rotten away down in the depths), their eyes might glow less on the surface but a lot under water? I don't know, you're the development team!

Might be a nice idea.

I don’t think they’re going to do that, it’s a rather uncreative idea. Not insulting your suggestion, I mean the way vanilla reused zombies like that.

Oh I don't disagree that Mojangs approach is terrible, it really isn't good, but at the very least this idea has some literal depth to it.
Two new advancements could also be added alongside it, and it could be used as a way of hinting at future content in the mod to discover. For example:
player gained the Advancement [A Curious Attack].
"Witness a Depth Hag climb out of the mud under a swamp."
player gained the Advancement [Oddly Familiar].
"Witness the Dreadful Peat Mummy resurface from the depth of the bogs."

Essentially making their attack a nod at the Dreadful Peat Mummy boss fight.
It also makes the Swamp Hags more of a threat in the, well, swamp part of the swamps...
Just a thought. :)

Climbing out of mud seems too similar to peat mummies.

Climbing out of mud seems too similar to peat mummies.

Well yeah, I literally said that... Nevertheless the event would only take place under water in mud, not on peat bogs.