I had a Borg origin outline wandering around my head once, and it was a little similar to the one we finally got in Trek Lit. I also gave the Borg a human origin, given that the Borg refer to Earth's location as Sector 001, and I also had a pre-Federation Starship thrown back in time to the Delta Quadrant.
No advanced species though, just a love triangle between a female starship captain (who would become the Borg Queen), a chief engineer, and a doctor that turns excessively tragic.
Marooned on a planet, surviving on a crashed ship, and hopelessly outnumbered by primitive inhabitants, the birth of Borg technology occurs when the engineer and doctor have to work together to save the life of the Captain, and create a cybernetic prosthesis to keep her alive when she's grievously injured in the crash. When it becomes clear that she can interface with the ship's systems following the procedure, and with a surviving crew too small to even consider repairing the ship, or getting, home, let alone establishing a colony and creating a civilisation, they realise that they can use the primitives as a labour source, by capturing them, implanting them and turning them into drones. Over the next few years, the surviving crew get implanted with control technology, and more and more of the primitives get turned into drones to service the civilisation that's now growing out of the wreckage of the starship.
But as the doctor and the engineer compete for the Captain's affections, and the rivalry turns sour, she begins using the control circuitry to control her own crew as well as the drones, and the collective is born. When a passing spaceship pays an interest in this abberation on what should be a primitive planet, the first assimilation occurs, and the Borg as we know it start their expansion.