Outside of the technical difficulties of pet customization I'm gonna assume they'd stil want the primaries to represent what they actually are titled, so Robotics should still be robots and Mercs still some form of military looking unit. Being able to change the type would be good though, Robotics could have ones selected from Longbow, PPD, Malta, maybe Arachnos Spiderbots. Mercs could select from ones like PPD special forces, Malta, Arachnos Wolf Spiders, and the like. Plenty of mob/thug style groups exist already, and there's a few options for Necromancy and Demons too: from the Circle of Thorns (minion Hordeling Berserkers; lieutenant Hellfrost Lord & Behemoth Master; and then the giant Behemoth Overlord for example) and the Banished Pantheon and Zombie zone invasion mobs could probably be made to work for Necro.