One day a swarm of self replicating probes will turn up and start building a Dyson sphere around our sun. The purpose is to power the alien equivalent of Bitcoin mining.

Another storyline I came up with is this: Imagine that an alien civilisation has the ability to send electromagnetic signals faster than light by some mechanism such as a micro wormhole but not ships and such.

They still want to destroy other civilisations before they become a threat. They are able to connect to some unsecured wifi networks and start posting on forums. The username: Satoshi Nakamoto