MCU vs Massive alien invasion
The entire Harvester fleet from Independence Day 1 and 2 invade MCU Earth. At the same the alien Battleship (2012) emerges from the Pacific Ocean. Los Angeles is invaded by the species of aliens seen in Battle For Los Angeles (2011). War of the Worlds (2005) walkers emerge from the earth on the East Coast. In the south, the aliens from Skyline (2010) invade.
This is the MCU in phase 3 (instead of attacking Earth himself, Thanos enjoys the view first, by seeing how this invasion plays out) Everyone assumed to be on Earth in phase 3, but only just appeared in phase 4, are present. The invading aliens, are allied for this mission.
Can MCU-Earth survive?