It is well-known that the MCU is tied together by the infinity-stones, most of which have already appeared,appeared; and they will play a central role in the upcoming Avengers: Infinity War.
They have all shown up in a space of ten years (a trivial span of time compared to their history) and all mostly on Earth, or in interactions with characters with close connections to Earth. (i.e. StarlordStar-Lord and Thor).
