Are there any hard rules aside from the few examples given and "use your imagination" to build your own artefacts?
No. Artifacts are not just magical items, they are also physical manifestations of the plot. As the Pathfinder SRD states,
Rather than merely another form of magical equipment, artifacts are the sorts of legendary relics that might become the focus of whole campaigns—sparking quests to recover them, fights against their wielders, missions to destroy them, and more.
As powerful objects that could drastically affect the story, they are meant to be custom designed. In the same vein, there are no constraints on what a GM can make an artifact do, and therefore there is no reason to have any rules about coming up with new ones (besides having a specific method of destruction, though even that could be ignored if desired).
Well technically if you wanted some boiler plate ideas for the powers of a generically useful artifact weapons/armor you might turn to Legendary weapons->Artifacts In Mythic Adventures.
Follow the rules as if you had a character take the mythic path ability for legendary items and then follow the steps to make it a minor artifact or major artifact.
But overall, artifacts should be used as your blank canvas as a DM to do as you need to create plot. Have fun with it. Create a theme. Hell, make the item intelligent and crazy. Use it to create an interesting narrative.