One great 'Imperium of Man Fanfiction' is 'The Emperor's Gift'. It offers a unique perspective on the Imperium, exploring the relationship between different Space Marine Chapters. Another good one could be 'In the Service of the Imperium' which delves deep into the lives of the Imperial Guardsmen and their daily struggles. And 'A Glimpse of the Imperium' is also worth checking out as it presents a broader view of the Imperium's politics and power dynamics.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'Imperium of Man Fanfiction' with various themes and styles. Another option is FanFiction.net, which also has a decent number of such stories. Just search for 'Imperium of Man' in the search bar and you'll find a bunch of them.
One interesting story could be an Imperium of Man crossover with Warhammer Fantasy. In this crossover, the Imperium's advanced technology clashes with the magic and medieval setting of Warhammer Fantasy. It could explore how the Imperium's Space Marines would fare against the Chaos forces in that realm.
Character development is key. When bringing together characters from the Imperium of Man and another universe, show how they interact and change. For instance, if an Imperial Guard meets a Jedi from Star Wars. The Guard might be initially skeptical of the Jedi's use of the Force, but as they work together, they come to respect each other. Also, don't be afraid to deviate from the established canon a bit to make the story more interesting.
Well, without more context, it's difficult to say precisely. 'Imperium' often refers to a large and powerful state or empire in a novel. It could involve a story of power struggles among different factions within this imperium, perhaps over territory, resources, or ideological differences. Maybe it follows the journey of a hero or a group of characters who are trying to change or survive in this imperium.
In a 'imperium in imperio novel', it might imply a narrative that delves into the power dynamics of a nested hierarchy. It could be a situation where a group, like a rebel organization or a cult, has established its own form of governance within the boundaries of an established empire in the story. This concept can create complex plotlines, as the two 'imperiums' - the larger and the smaller - may interact in various ways, such as through conflict, cooperation, or a tense co - existence.