A better question would be, could the Borg assimilate another machine race like the Cylon Empire?
Could be very interesting seeing as both are quite dominate in their respective series as a powerful force to overcome. The Borg might have a technological advantage over the Cylons in a battle, but the Cylons numbers game with Baseships and hordes of Raiders might surprise the Borg and they could be overrun by the Cylons. The Borg Queen might find her match in the Cylon's Imperious Leader who's origins are far more darker than hers.
The Borg facing other movie/TV series villains would also be a great
'what if' scenario to play out as well here.
The Borg Vs The Cybermen? (Dr Who)
The Borg Vs The Daleks (Dr Who)
The Borg Vs The Vistors ('V')
The Borg Vs The Peacekeepers (Farscape)
The Borg Vs The Commonwealth (Andromeda)
The Borg Vs The Goa'uld (Stargate SG-1)
The Borg Vs The Wraith (Stargate Atlantis)
The Borg Vs The Replicators (StargateSG-1/Atlantis)
The Borg Vs The Ori (Stargate SG-1)
The Borg Vs The Federation (Blake's 7)
The Borg Vs The Shadows (Babylon 5)
The Borg Vs The Drakh (Babylon 5/Crusade)
The Borg Vs The Hand (Babylon 5 - Legend Of The Rangers)
The Borg Vs The Imperial Empire (Star Wars)
The Borg Vs The Necromongers (The Chronicles of Riddick)
Now those kind of confrontations is something any fan can debate at any great length here now isn't it!
KJ
