You say it wasn't intended to be recurring but in the same episode they Gunan is talk with Picard and she says "now that they know about humanity..." Picard finishes his statement, "they'll be coming."
That's not what I mean by recurring. My other examples, Andorians and Cardassians, also had subsequent appearances in the shows they originated, but were never major players, nor were they intended to be, until ENT and DS9; and for the Borg, not until VOY.
Then there is the episode before Q Who about the neutral zone and bases that had been destroyed...sounds like a set up and recurrence to me.
version of the Borg was scrapped by the time Q Who
came along. The Borg that we see in Q Who
were created for Q Who
only. They were never intended to replace the Ferengi as TNG's recurring bad guys as the original Neutral Zone
concept was. That concept, along with several others from that era, were aborted by a writers' strike. Those Borg were supposed to come from the Alpha Quadrant, not be introduced by Q 7000 light-years away. The whole point was for Q to show them something terrifying that they had never encountered before
, not "those guys you encountered in season 1".