So the question has to be asked: What about this?

A Star Wars/Warhammer 40k crossover has to include the battles of evil vs. slightly more justified evil and feature a large Imperial Navy cathedral armada going up against the Death Star (presumably with its own array of Star Destroyers). Now I have put some actual thought into this.

• The Imperium is not likely to be terribly fazed by the Death Star, thanks to confrontations with giant horrific superweapons being known as “Wednesday” over there.
• If they cannot overpower it via masses of lances, nova cannons, and whatever other exotic tricks they’d undoubtedly pull out (ie psychic lashes, vortex torpedoes to just shunt it into the warp, etc…), for the main Imperial Navy it would be difficult to duplicate what the Rebel Alliance did. The reason simply is because Warhammer 40k space fighters are substantially larger than their Star Wars counterparts, and their ordnance is the same.
• However, moving in for a pinpoint strike on a difficult but decisive target is the exact thing that Astartes were made for. So trench runs of Thunderhawks, vacuum chainsword fights, and tossing a melta bomb down the port would make for a fitting story.