A lot of oldtimers from the era of the original Elite game still remember the menace of the Thargoids and miss them in Frontier. Nobody has seen a Thargoid in Frontier yet but in an interview David Braben shall have told that there is a single Thargoid ship in a system far out in the galaxy. I personally think this is only a trick to keep us playing Frontier until add-ons appear. But who knows?
If you fly bombing missions to federal bases you can sometimes see two big violet octagonal ships that look just like the good old Thargoid ships but they are not disturbed by your approach. Perhaps the Thargoids sleep for a few hundred years after the exhausting fight with mankind?