2. The Fault/The Cancerverse
When Erik Selvig has gone a little bit insane in Thor: The Dark World, he is housed in what appears to be either a care home for the elderly or some kind of mental health institution. Whilst in there, he gives a rambling talk - utilising a chalkboard - to tell his fellow residents about cosmic abnormalities and he uses two shoes to visualise his point (smashing them together to represent the coming together of the nine realms). It is at that point that Stan Lee's usual cameo occurs, as Lee is revealed to be a fellow resident and he proceeds to ask "can I have my shoe back?" However, it is what is going on in the background that concerns us here. On the chalkboard is a diagram of what appears to be the World Tree (which we have already seen on a couple of occasions in the Marvel Cinematic Universe) and a number of scribbled words depicting locations and calculations that Selvig has jotted down. Amongst them is "The Fault", which was a rift in space-time formed at the end of "War of Kings" and a major plot-point in the comic book arc "The Thanos Imperative". Beyond the Fault lies the universe known as the Cancerverse - a universe where death itself is extinct. This could be a meaningless Easter egg or it could be a clue to the nature of Thanos' future involvement in the Marvel Cinematic Universe. Either way, it's cool.