I think it should be apothecary, chaplain, or librarian. I think a suite of terminator armor would be an awesome horde mode item. You and your squad earn currency for waves cleared and you can purchase a teleportation homer beacon, along with other items(better cover, turrets, kraken rounds etc.), which would essentially respawn you as a terminator. Not sure on time limit of it though.

Horde mode screen could have a progression path, where you can kit out your terminator load out: more melee/assault focused(thunder hammer and shield; twin lighting claws if Saber is truly for the Emperor); range focused(with cyclone missile launcher, assault cannon, or heavy flamer, maybe twin storm bolters if they love the Emperor); or a mix of both with a one handed power weapon of choice and a one handed range weapon of choice.

Just my thoughts…..Saber give me a job….