The characters are a big part of it too. You can have classic Star Wars characters like Luke Skywalker interacting with Call of Duty - like soldiers. Or, you could have a new character that is a blend of both universes, like a soldier who was trained by a Jedi and now uses those skills in the Call of Duty - inspired battles. This mix of different character archetypes from two very different franchises makes for some really engaging stories.
The settings are also very unique. You get to see the iconic Star Wars planets and locations, like Tatooine or Coruscant, as the backdrop for Call of Duty - style battles. This creates a really interesting visual in the reader's mind. Also, the technology crossovers are cool. Imagine a Star Destroyer being used in a Call of Duty - style mission, or a COD - style drone being used in a Star Wars battle.