Well. The Borg used
to be awesome. They're an awesome idea, certainly, but as we've all seen, badly misused.
They could be brought back, but the key is, you've got take some advice from Aretha Franklin and treat them with R E S P E C T.
That means: the Borg almost never can be defeated, unless Starfleet is being incredibly clever. Our jaws should hit the floor, they're so clever. Short of that, the Borg wins. Most of the time, the only solution is just to run away.
And assimilation is P E R M A N E N T. No go-backs. No treating assimilation like a dress-up party. The next time the Borg is brought into a story, some popular character should be assimilated and it's permanent. Someone like Chekov. Something that will punch us in the gut, make us really understand, shit is getting real.