I think that if I was to make a team for 40k it would have to be the Talien Dog Warriors, or SPACE LIZARDMEN! And I think they role they would fill is the jerks. So there are a lot of different ways to play armies, Imperial Guard can be all men blobs or they can be, a load of tanks, necrons are slow and purposeful, and marines are meant to be tactical.

Now some people may disagree with me and say there are plenty of jerk people, most everyone infact, however there is no jerk way of playing. Some people may say Eldar players, but there is no jerk way of playing team. So Eldar are Glass Cannons, Orks are brute force, Tyranids are don’t break cause we are hive mind, Dark Eldar are the super fast team.

However a better way of saying it would be this, that Taliens are closer to being cold, both literal and metaphorical. So, their objective cards might say something like this, if you control objective 1 gain a victory point, if your contesting it but control due to being a troop you gain two, if you have one model contesting this objective gain five victory points. Or another might be if you have three or more units fire upon a single unit gain a victory point. And finally maybe a rule like this, if you win in combat, and gain a sweeping advance, instead remove the model leading the unit, that unit has minus three to it’s leadership until end of turn. It’s not that they wouldn’t kill them, it is because they want to see them run in fear of them and so on.

But In reality I don’t think that is what 40k needs at all I think they need a new ‘army’ but it to not be an army at all. Just called something plane like Outer system mercenaries. Every single unit has an alliance chart, but they never ally themselves with the Imperial forces, and never with Chaos so that other armies have a year round thing to look forward to. Tau get greater allies, Dark Eldar get the weird things, Eldar get the other older races, Orks get the really brutal fighting guys. and usually they would cross two or maybe more of those alliances.

Like you could see a weird, brutal thing being Ork, Dark Eldar allies, but allies of convince with everyone else. Or an old race allying with both Eldar groups as they see them as just Eldar. Or a race that see the Impeirum as bad, but still a good people so they ally with the Eldar and Tau, such as like an Exodite force or whatever.

I think if I was to make another force it would be exactly like @brennon said, an pre-crusade force. Which I would call something like the lost and the Imperium does pick up these kinds of people. Take a look at Catchan that world is primarily kept going by off world shipments. But they do that because they like the people that come from their they make great soldiers. You can have old school techno barbarians fist of the north style. All sorts, and this allows you to have a both Chaos and Imperium allied force because they work for whoever is protecting them, Chaos Space Marines don’t seem that different from Regular or Rebel Space Marines to them.

And finally the other thing I think I would do is the Allies Imperialus. These would be the new book for the Inquisition, Rouge Traders, and other non-standard Imperial forces. Like the Adptus Arbites and so on. But yeah that is just me spit balling nothing solid.