Imperium of Man vs the A-Wave
The entire A-Wave restores itself to full force and resources and travels to the Warhammer 40k verse in hopes of conquest. Deciding it better to focus on one race at a time it begins to target the worlds of Mankind dropping billions of its foot soldiers to swarm the nearest humans planets as they begin their attack. Of course Mankind attacks back and an all out war begins.
VS
An all out War scenario between Warhammer 40k's imperium of Man and Marvel's Annihilation wave. Imperium has a fully healed Emperor and the Annihilation wave is at it's peak with the Galactus gun and Thanos as it's strategist. Neither Army knows anything info on the other in advance and will only discover things as it goes on.