My apologies if this has been brought up before, but it just occurred to me: why didn't the Borg create the temporal vortex in their own galaxy, thereby precluding any possibility that they would be interfered with?
I'm assuming they traveled from the Delta Quadrant. Couldn't they have just made the same journey...but in 2063? If they had done this, wouldn't this have assured they met no resistance when entering Earth's orbit?
What if the goal of the Borg wasn't actually to assimilate Earth in 2063, but was in fact to insure the success of first contact with the Vulcans, the formation of the Federation, and the creation of a vast technologically advanced society for the Borg to assimilate at some point in the future? 7 of 9 said that first contact was successful precisely because the Borg traveled back in time and the Enterprise intervened in stopping them and in helping Cochrane achieve his warp flight, and that the Federation ironically owed its very existence to the Borg. They purposely lost in the short term to win in the long term.
The same rationale could explain why they only send one cube at a time, why they enter Federation space a long distance from Earth in order to give its forces time to muster a defense even though a transwarp conduit exits right at Earth's front door, why they give their enemies easy opportunities to defeat them (don't attack boarding parties unless they're a threat, don't cut off captured drones from the Collective so they can be used against you, blow up if you're put in a sleep cycle, give Picard a connection to the Collective again so he has early warning of your approach and knows what vulnerabilities to exploit...).
Until humanity posed a threat to the Borg's very existence with Captain Janeway's actions (ironically again caused by the Borg increasing humanity's technological level) the Borg were simply fattening the hog before the slaughter, improving Earth and Federation society through conflict and adversity until they were ready to be assimilated at the biological and technological peak of their existence. By intervening in 2063 they created the Federation. By intervening in 2366 they destroyed a bunch of old ships and spurred the development of newer, more advanced ships and technologies. Unfortunately for them, their human lab experiment developed to the point where they threatened the Borg themselves, which is why you get the effort to destroy humanity completely in the Destiny
This kind of temporal planning would require a technology that would allow them to view probable futures based on simulated changes to the past before actually making the changes, similar to the technology employed by Annorax and the Krenim. This would tell them that if they did action A, they would have a 83.62749% chance of achieving outcome B, but it would also be an inexact science by its very nature that would allow for unforeseen consequences (like the Federation eventually becoming powerful enough to defeat them). That ambiguity would also leave time travel as something that was still a dangerous tactic to employ unless you were reasonably sure of an outcome in your favor, so it wouldn't be used often even by the Borg.