This topic has not been talk about to much, and I am hoping it might be soming added down the road in STO.
This is the Borg as we know it:
They are a collection of species that have been turned into cybernetic organisms functioning as drones of the collective or the hive. A pseudo-race, dwelling in the Star Trek universe, the Borg force other species into their collective and connect them to "the hive mind"; the act is called assimilation and entails violence, abductions, and injections of cybernetic implants. The Borg's ultimate goal is "achieving perfection".
You have a common misconception about the MU. What many people dont seem to realize is that the only faction that is truly opposite in the MU are the humans. All the other differences between the MU and the normal universe are a result of humans being evil and that changing the way they interracted with the other galactic powers. There wouldnt have been any Klingon/Cardassian alliance if they didnt have a reason to become allies in their fight against the Terran Empire. Long story short, the motivations and origin on the Borg in the MU is no different than in the normal universe. The only difference would be what happened after they encountered humans.
You have a common misconception about the MU. What many people dont seem to realize is that the only faction that is truly opposite in the MU are the humans.
Not quite. Mirror Kira Nerys was completely loco - a hedonistic, bisexual nymphomaniac prone to violent mood swings and drunk on power - more or less the exact opposite of the Prime Universe Kira who became Kai.
Also, Mirror Odo, rather than being devoted to the cause of justice, only cared about keeping order and maximizing efficiency in the ore-processing plant he oversaw. From what little we saw of him, he was cruel, sadistic and unreasonable - again not all like his Prime counterpart.
"The Snowmenz Are Coming! The Snowmenz Are Coming!!"
The Borg in the MU could be an infectious organic disease that infects both organics and machines, mutilating them to whatever function it needs. They might be like an intelligent animal or some sorts whose only desire is to spread itself all over the place. And it apparently spreads by using technologically advanced space faring races as its vector. And it is apparently intelligent enough to imitate distress calls...