I vote for dropping the fire/elec immunities, and agree with damage reduction 10/magic and bludgeoning and fast healing 15.
I've used juggernauts several times, but they were always advanced by HD. Still, they seemed on-par for the CR following the usual advancement rules, so I have no problem with their current CR. And I agree that the mimic version should be 1 less than the construct version.
That sounds pretty good. I'll let you fill in the tactics, description, and flavor, since these are your babies.
The description is a bit more problematic. Are we keeping these a bizarre mimic-golem hybrid like the original monster, or are they just regular living creatures than breathe, eat and breed?
I'd prefer the latter. Golem-hybrids really make little sense, even for nonsensical D&D monsters.![]()
Or we could just give both origins, and leave it to the DM to decide.