[@AdvancedJ3lly]

as your the DM, we honestly are going to be doing whatever you want us to do. If you have an easier time thinking up an idea against the dominate (which would be my preferred enemy behind fighting some chaos cultists) or putting down some other sort of rebellion in the ass end of the Imperium. 

Putting down a chaost cult incursion is also an option. An imperial world overrun with one or more cult influences and now everything has gone to shit. You still get a lot of personality because, those cultist faces are human. There is all manner of devestation and crazy shit to encounter in chaos controlled territory that will make characters vomit or lose their sanity. 

Or maybe we come face to face with actual lost and the damned regiment. that would be a good mind-fuck for a lot of soldiers.

As for regiment. Ya, elysians are cool. I mentioned Brimlocke dragoons(or any mechanized regiment) because that also gives us some options. We can ride around in a chimera and assault enemy lines, do recon, etc. simply having a vehicle for deployment, redeployment and fire support keeps a lot of options open.