You read my mind, I have a knack for coming up with concepts that are later implemented into the game without my input.
I predicted Genesis Part 1 and the upcoming Arrat Prime Expansion.
I do like your concept. I could see a TEK saddle for this creature as well, it projects a shield forward that can be shot through on one side, but will negate the damage incoming from the other, it decays slowly & moves with the crab, the shield is highly resistant to ranged attacks, but it suffers greatly against melee. Even a Survivor with a Machete can disable the shield within minutes, but turret walls will struggle to put a dent in the health of the shield.
I could see it also appearing in the underground world, the water is perfect for it.