[Oath of Moment] the hardest part of coning up with a 40k story is trying to decide which factions to use as the bad guys

There are so many good options

The Imperium itself being pretty high up on the list

the true answer is all

the only people who are not bad guys are the main characters, and even then only from a certain perspective

That would be a messy campaign

The initial setup, at least, should seem pretty clear cut

“An Ork Waaagh! has entered the system and the only way to stop it is to assassinate the warboss”

That's always a good start

“Eldar witches have been seen shadowing our forces and their webway access site needs to be located and destroyed”

nothing like starting a campaign with lots of dakka, only to discover intrigue, subterfuge, horrible scandals
and perhaps more dakka

“The Archenemy has corrupted the planetary governor.”

Old Wolf Winter : needs more dakka

always needs more dakka

(ASMR) 1 hour of Dakka for a calm, relaxing sleep (B...

"An ork warboss entered the system, but when we mustered forces to oppose him we found that there were also chaos and genestealer cults on the planet fighting a secret war. Also, we found a moon with a bunch of Squats on it so the Sororitas purged it but now there are these 'League' guys here with a bunch of T'au.

Then the Necron Tombships started appearing and broadcasting demands to surrender the system to them and vacate their property, and while we haven't seen any eldar or Tyranids proper yet, but we know they are on their way."