I agree there should have neen a follow up to "Conspiracy."
There is another Next Generation episode...and I can't recall the title at the moment. It was a spoof of contemporary alien abduction lore. Riker was kidnapped off the ship in his sleep and experimented upon by aliens, and then was put back in his quarters afterwards. The crew discovers that aliens from sub space (if I remember correctly) are responsible, and the aliens send a probe (or something) into normal space at the end. Neither the creatures nor their motivations are identified. This episode seemed ripe for a follow up, unless the producers intended to leave the questions unanswered.