Skyrim's world practically begs to be crashed into others, but some mashups stick around more than others. The 'Dragon Age' crossover is everywhere, and it makes total sense—both are dark fantasy RPGs with grey morality, ancient magic, and a playable hero who can be anything. You get a Grey Warden or an Inquisitor stumbling through the Pale, or the Dragonborn showing up at Skyhold and completely derailing the Breach plot. The tone matches almost too well.
More interesting to me are the weirder ones that somehow work. 'The Witcher' fits because Geralt’s monster-hunting cynicism bounces perfectly off Skyrim’s often-serious Nords. I’ve seen a few that throw the Dragonborn into 'Game of Thrones,' usually with the political schemers in King’s Landing having zero clue how to handle a walking natural disaster who shouts dragons out of the sky. Those can be power fantasies, sure, but the culture clash is fun.
The real niche gems, though, are the ones that shouldn't work but do because someone committed to the bit. I read a 'Star Trek' crossover once where a damaged shuttle crashed near Winterhold, and the crew treated magic like a new branch of physics. The author put so much work into how a Vulcan science officer would analyze a soul gem. It’s those off-the-wall blends that stick in my memory more than the obvious pairings.