IMHO one of the easier ways would be indeed to have the Borg assimilate a species whose technology could give them a very significant edge over StarFleet - be it a way to cut through Fed shields like the Dominion's phasers did at the beginning, a technology allowing them to beam nanites through raised shields, a phasing cloaking device, allowing Borg cubes and drones to pass through matter and regain their solidity whenever they want, or anything cleverer. Maybe the Taurhai or the Dominion (weakened after the war) could do the trick, otherwise any Delta Quadrant species of your choice (Krenim ?).
After this assimilation, however, the Borg would be made aware that the Federation will soon have reached a high enough tech level to counter this advantage (either through statistical estimations, or by some Borg having travelled to the past to warn them - why would Janeway be the only one to do that ?
). They then proceed to assimilate the Federation as soon as possible to prevent it from inventing this counter tech, since they're the most advanced power in the galaxy in that area. Once the Federation assimilated, no one will be able to oppose this new technology - and the Borg.
The crew could then either help the species to avoid assimilation, prevent the Borg from learning of the Federation's advance (if they did because of another time travel), or on the contrary bring it back from the future to counter the Borg's invasion force in time. Or even try all of these courses of action, should one of them fail.
Lots of challenges and time travel anyway - the Guardian will be needed 
An alternate idea would be the Federation to be the one initiating the attack - but having failed. This would require the Borg to have tried something very dangerous to the Federation, to the point of bringing it to launch a preemptive strike.
This could also happen if a Romulan (or whatever) spy infiltrated the Federation at a higher level and influenced its politics toward this strike by falsifying data to make StarFleet think it could be won. Such a premise has the advantage of avoiding putting the players directly against the Borg, since they'd only need to foil the spy's plot to correct the timeline.
Last edited by C5; 03-12-2002 at 05:22 AM.
"The main difference between Trekkies and Manchester United fans is that Trekkies never trashed a train carriage. So why are the Trekkies the social outcasts?"
Terry Pratchett