I've been assuming Armada Thrust or Cybertron Crumplezone for the Left Arm, but the Armada comics featured a classic G1 character as an inter-dimensional herald of Unicron...
And he has had a color scheme that's a a pretty close match for the left arm:
Maybe the left arm is a double homage to Thrust (who ended up working for Unicron in the Cartoon) and Thunderwing (who served him in the DW comics) and looked very similar to Armada Thrust in the Marvel comics? Or is that too convoluted?
Maybe the whole point of this combiner is represent a
multiversal combiner - sort of a Nemesis Nexus, made of Transformers culled from across multiple Universes. The right leg could be a bizarre take on Cybertron Soundwave or Beast Wars Neo Elephorca?
Though, if they went anywhere in the servants of Unicron direction, it's too bad they're not doing the
Four Horsemen.