There is a strong thread running through Warhammer: that of corruption from within. This gives it a thematic strength that is rare in fantasy settings. Where most such settings tend to be exercises in imagination, Warhammer feels like it is genuinely about something - the conflict between Man and Self.
The first and most important element of this is, of course, Chaos - a force which corrupts from within and thereby destroys. Each of the major Chaos Gods tempts mankind in different ways, drawing people away from their essential humanity and transforming them into something baser, by playing on their fears, hopes and instincts. Khorne does it through awakening rage and bloodlust, Nurgle through despair, Tzeentch through the lust for knowledge and Slaanesh through plain old lust. People are drawn to their worship from an inherent weakness, a chink in their armour, and are taken - like those who cry out to Nurgle to protect them from plague, or who give themselves to Tzeentch in return for hidden knowledge. Chaos is a constant threat which seethes below human society; everybody must be on their guard lest they become tempted. This sense of danger at the roots of mankind is personified by the Skaven, a clandestine race of rat-men who live right under the foundations of human society, unknown and unsuspected but constantly gnawing, spreading disease and pestilence and gradually gathering their strength for the moment when they strike.
I likened the Empire to the USSR under Stalin in my post the other day, and I like that analogy - the difference, of course, being that the threats which faced the USSR were mostly made up and trumped up, whereas those facing the Empire are all too real. By coincidence at the moment I'm reading Orlando Figes' The Whisperers, a massive collection of oral testimony from people who lived through the Stalin years. The sense that foreign spies were everywhere and that anti-Bolshevik 'cosmopolitans' and 'class enemies' were lurking in the shadows around every corner pervaded the Soviet regime in its early years (though the real danger for most people was in fact the NKVD): the struggles which families of those accused of being 'class enemies' went through could well be compared to those of the familes of traitors given over to Chaos. Warhammer is like a 'what-if' scenario which imagines what a world would be like in which the lies of totalitarian dictatorships were actually true. Except transported to an incredibly baroque and blackly humourous fantasy setting.
This sort of thing is something I'd like to play with with Yoon-suin. Not the straight plundering of Chaos or Skaven or class-enemies, but the creation of some sense of an all-pervading and ever-present danger threatening to consume everything from within. Something to make players feel as though they have to keep a constant guard on their own behaviour lest they take a wrong step somewhere and find themselves slipping into something which might endanger their immortal soul. But which at the same time wouldn't mean an ending, just as the man who gives himself to the pleasures of Slaanesh loses his humanity but gains something Other.