It's funny, I feel like I've read fifty different versions of their dynamic. The most common thread has to be 'what if they were both a little less repressed?' Canon gave us all that simmering tension in the office, but so many writers just can't resist pulling the trigger.
You'll see a lot of stories that start from a small canon divergence—maybe Margaret stays late to finish a report, or Mordecai finally snaps and asks her out for a coffee that isn't just about work. The early stages are almost always awkward, with both of them tripping over their own professionalism. It's a slow dismantling of their workplace personas.
Where it gets really popular, though, is in the 'established relationship' fics that skip right to the domesticity. People love imagining them navigating a shared apartment, with Mordecai being impossibly neat and Margaret secretly leaving her mugs everywhere. The conflict shifts from 'will they or won't they' to 'how do these two very particular people build a life together?' The appeal is less about the chase and more about the quiet compatibility underneath all the stiffness.
A whole other branch leans into the angst potential. What if someone from Mordecai's past showed up? What if Margaret got a better job offer elsewhere? Those fics explore the inherent instability in their bond, the fear that the other person might just be too… much. It's less fluffy, but it feels grounded in the characters' core anxieties.