Chapter 2 - Warhammer 40k
Many of these ideas involve someone being transported into Warhammer 40k. In addition, they probably shouldn't be taken too seriously.
Commissar of Dangerous Regiments
The protagonist awakens as a newly formed commissar, in the universe of Warhammer 40k, being taken to meet his regiment, only to discover that the regiment he will command is composed of forces from different regiments that are troublesome to deal with, such as Catachan Jungle Fighters, Death Korps of Krieg, and Savlar Chem-Dogs.
Now he has to lead the best, baddest regiment in history while trying to avoid being killed by his enemies and allies.
Note: If you know of any problematic regiment, please let me know the name so I can look it up.
The Most Hated Chapter Master.
This time the protagonist awakens as the chapter master of a Space Marines chapter, at first it's not that bad, he could have been reincarnated in a worse place, that is until he discovers that he is in the body of the Marines Malevolent Chapter Master, the chapter of Space Marines most hated by members of the Imperium and considered by the Inquisition to be excessive in their use of force.
Being the Chapter Master of the Space Marines who embody the phrase "the end justifies the means and there is no such thing as innocence, just different degrees of guilt", our protagonist needs to change his chapter to something honorable, and remove the bad reputation his chapter has in the Imperium.
Warp Entity
The protagonist becomes a Warp entity, not affiliated with any God of Chaos, instead he tries to help those lost in the Warp, guiding ships, preventing Daemon invasions, helping Psykers and beings that are not warp entities, but even so, they roam around the Warp, which is a lot by the way.
Note: This story is based on an existing story called Spaw of the Well, can be found on the Spacebattles website.