Or in the winter theme like the dlc of the first DOW, or hindsight with the black templars.
You choose the environment

Maybe some defense with the fisty bois (imperial fists) against a iron warriors siege (i know they are not in the mod but you can use the standard heretic marines) there is a lot of matterials in the mod and baseline game to make a fortress or better remember the times in lore when the eldar and humans joined forces there were some maybe you can do that so that way we can have protagonists that are something else than humans wile still not being disgusting xeno propaganda or even better the defense of terra there is some 30k stuff you can use for that and no one tried it not even outside the game imagine defending the emperors palace in trenches holding of hordes of chaos with astartes and guardsmen.