For Operations, I wouldn’t mind if Orthus Offensive counted as an operation map, for the sake of penance completion (I really don’t want to play Rolling Steel another 25 times).
I would also very much like to fight aboard a ship such as the one seen in the finale of, what was it, consignment yard? Or a return to the Tutorial ship. Or repelling enemies off the Mourningstar. All three of those would be neat settings for operations.
As for Raid Bosses, similar to The Twins, I have two ideas I’d like to share.
The first, is we mount an assault on a Admonition stronghold. We climb a tower, but when we get to the top all that’s waiting for us is a Beast of Nurgle. But, after killing it, a Rotfly bursts from its body and begins to fly around the arena shooting goop at us as we fight off mobs, before landing to give us a chance to wail on it. After taking enough damage, it flies away, and we think “Ah, that wasn’t so bad” as we fight off another wave, but then it comes back with Rhina-karkin-karnack riding atop it all mutated to shiz and it’s the same fight again except when the rotfly lands Rhina dismounts to wail on US. The secret version has Rhina come out of the Burgle too, starting the 3rd phase early and for twice as long. The arena also shrinks as permanent creep oozes out around the edge.
The second, is we’re sent out to investigate missing strike teams that all vanished trying to collect some valuable intel, but when we get to said intel it turns out to be a trap, and the missing strike teams have turned traitor. So we have to fight off 4 corrupted rejects, each a twisted reflection of our operators, and with different strengths and weakness’ to exploit (Zealots are fast, and can be easily separated, psykers are glass cannons who will eventually explode on their own, veterans are weaker in melee, and Ogryns are big and slow, etc). When one dies, another reject hops in to take its place, sporting a different weapon/combat ability. This goes on until we exhaust their reinforcements (We run out of fun variants to fight). The secret mode involves having to fight more at once (now randomized), or all at once, depending on how many reinforcements there are to begin with.
Alternatively, we could fight each of the 4 reject-types one at a time, alongside standard enemies, and the secret mode is we have to fight all 4 at once.
Alongside their obvious mutations, you could also randomize their cosmetics. Just for fun.