Board Thread:General Discussion/@comment-1031724-20160803045138/@comment-1031724-20160804042019

Actually I forgot. There is a rainbow in the MCU. One with six keys just like the rainbow that damned Tirek, and powerful enough to destroy even Thanos.

The Infinity Stones. All six of them are colors of the rainbow. We've seen blue, red, purple, and yellow, and we know from the Collector that the other two will be orange and green.

My new guess, the Avengers take the Infinity Gauntlet somehow (maybe Ant-Man steals it), then use it to blast Thanos with a rainbow. Then, in the postcredits scene, Thanos is revealed to still be alive, and the series ends there.