I would assume that if for some reason the Borg has appeared during Star Trek The Original Series, they would appear quite different. The budge for TOS was very low compared to Star Trek today, so I would assume the Borg wouldn't have all those "High Tech" implants. Most of the aliens in TOS that were encountered by the Enterprise looked human as well, so perhaps a TOS version of the Borg would look like a regular person with a couple of flexible tubes? Obviously some flexibility is allowed due to the Talosians from the first pilot, but what about facial constructs? Maybe just the eye laser and that's it I'd assume. The borg exokeleton would likely just be a cheap plastic torso covering. What do y'all think?