Popped in my head and so now I have to vent it here.
Clan Moulder Mutator
Vaguely based on and resembling Throt the Unclean, this is a fairly large (by Skaven standards) special who moves slowly. He’s not that threatening alone, but can enhance nearby enemies to become stronger, faster, and tougher - think a Wargor’s banner, but only on one or a few enemies. Like a Gas Rat, he’ll attempt to stay at a reasonable distance from the heroes, but approach close enough to “enhance” a Skaven unit aggroed on the heroes with Warpstone Injection.
With lower-tier enemies, he can enchance several at a time, but he’ll focus on the highest-tier enemy available, and if he picks an Elite or more he’ll affect only one at a time.
I’d say make the buffs vary; higher HP and more damage would always be an effect, but they might get a damaging aura, a slowing aura, an aura that makes them poison on hit, or perhaps they’ll explode after a short time.
Most excitingly, the Moulder Mutator can buff monsters, too! =D And he’ll prioritize them the highest of all!
Unlike Gas Rats, the Moulder Mutator can defend himself in close combat, and has three arms that he’ll attack with in a pattern (like a Berserker, very quickly, but with good timing you can interrupt him). His armor type is Infantry, and his HP is high, but his stagger resist is low - the best way to deal good damage to the lardball will be to shove him over.
His weapons will be a bonesaw and a needle - if he hits his last combo attack, he’ll inject the hero with some warpstone serum, which could have a variety of debilitating effects, like blurring the screen, causing a DoT, debuffing their damage, causing a Curse-effect of reduced health for a time, or who knows what else.
Moulder Mutators will announce themselves with their horrible cackling.
Looking on the Clan Moulder wiki page gave me another idea that seems thematically perfect with the game . . .
These are Norscan marauders either crazy enough to turn themselves over to Clan Moulder for “improvement” or else have failed in some way and been sent to the Skaven as punishment. Hooks and claws and chains have been surgically attached to their limbs, which are dangerous melee weapons, but they can also use hooks on chains to try to grab heroes and pull them towards them - I imagine this as having a long wind-up, and not the greatest of range - perhaps comparable to the damage range on a Warpfire Thrower. It would work similarly to Roadhog’s hook in Overwatch, and a well-timed dodge will avoid the grab.
I imagine them as being rather agile, with a back-jump similar to Ungors, and a double-flurry melee attack with their arm cleavers. Their armor type would be Berserker, and they would be announced by the rattling of chains.