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If you remove the rider and mabye a couple of limbs to get it to the standard six limb Tyranid thing and mabye a small head swap. What do you think?

The top carapace/shell does indeed resemble Tyranid armour. When I first saw these guys I too thought about their potential as a Tyranid conversion.

A head swap would indeed be in order as well as some filing and GS to smooth out the boils and blisters and to steer the model away from its Nurglyness.
I'd also consider replacing the limbs with Tyranid ones but the present limbs could still work.
And for bonus points, a couple of well placed Tyranid carapace bits, perhaps on the abdomen.

Anyway... that's my vision for them