In my headcanon, the Borg eventually reach a truce with the Federation, and over time eventually become full-fledged members of the Federation. That's CRAZY right?...Is it? I mean, the Federation warred with the Klingons and Romulans, and look how those relationships changed over time.

How about the Borg's willingness to join? What we've been seeing over the years is that the Borg adapt. Their willingness to adapt had been established from their very introduction as a faceless hive-mind. Over the course of the franchise, they've experimented with individuality with Locutus, Borg Queens, becoming so infatuated with individuality that they even dispatched 7 of 9 to live amongst Starfleet to investigate directly, and then instead of efficiently assimilating her to gain her knowledge, they choose not to re-assimilate her so that they could ask her about that experience and avoid corrupting that knowledge via assimilation. Why is the Borg so interested? The Borg found that Federation individuality had repeatedly repelled Borg invasions when Borg calculations indicated that they should have won, and even after re-adjusting for past failures, the Borg still found themselves stymied in encounters with Starfleet. The Borg were even saved from total extinction by the ingenuity of individual creativity and a temporary alliance with a Starfleet ship. That is a huge motivator for the Borg to re-assess their approach and look for a new way to adapt to prevent their vulnerability to a similar event in the future.

Would the Federation be open to it? Like I said, they've allied with past enemies before. Ex-Borg members of Voyager served with distinction. Borg tech has proven invaluable to Voyager's return. Most importantly, Borg Drones are not undead zombies! Assimilation is a reversible condition, and that means that instead of hating the Borg for killing their loved ones, the Borg ARE their loved ones. Moreover, Borg assimilation is a weapon of mass diplomacy. Chakotay found that the hive-mind allowed warring alpha-quadrant races to all live in harmony in the Delta Quadrant, and losing access to the hive-mind allowed their old destructive conflicts to creep back in, and ultimately they reinstated a local hive-mind to regain peace. Chakotay joined that hivemind and came away from it with unparalleled understanding and empathy for the other members of the collective, and an overall positive experience, and he disconnected with immediate recovery and no ill-effects!

That is a game-changer, it allows the Starfleet to show up on the door of a new alien race, and those aliens would naturally be cautious, suspicious, mistrustful of the Federation's intentions. First contact is extremely dangerous. An alliance with the Borg could allow Starfleet to establish first contact by saying, "We come in peace", assimilating the alien envoy, and then the alien representatives would know that Starfleet truly and honestly means to "come in peace", casting aside all suspicion of ulterior motives. Starfleet then disconnects the alien envoy from the local hivemind, and then those envoys can go home and sing Starfleet's virtues to the rest of their race.