What is a legacy? Specifically, when DC says that Legacy is something that has been missing in its books since its most recent relaunch (The Nu52), what does that mean to older readers and to newer ones that may not understand what that means? Why did the more endeared fans reject the changes that the Nu52 brought but when Rebirth, the launch of this new reboot but not really a reboot, arrived it is met with resounding cheers? In a nutshell, it is because of the rebuilding of a respect for legacy that the editors at DC have finally shown. Taking a look throughout the Rebirth special, there are signs of Legacy everywhere (Also SPOILERSPOILERSPOILERSPOILERS!!), from Ted Kord returning to possibly act as Jamie Reyes mentor to Ray Palmer dragging an unsuspecting Ryan Choi into the world of shrinking, it is about the last generation helping the next and in a few cases fixing some missteps from before Flashpoint, and in its wake The Nu52, ever happened.
The question remains though, “why are these legacies important?” For the sake of the story they are touchstones for a lost Wally West to try and find an anchor back into reality, for the audience, they are a look into where this new continuity is going. The connections they chose to use for this story are not just random tandems, they chose heroes that many older fans felt never got the respect or the send-off they deserved, almost as an apology and a promise to do better this time with them. Looking at the focus on mentors and students you find that the possible future of the DCU lays in the power of history. That for a majority of their heroes the person behind the mask can be changed without too much hassle (the numerous Earth based Green Lanterns, the Flashes that existed before even Jay Garrick got hit by lightning, Blue Beetles of all kinds and even a couple of Batmen), that isn’t to say it is done painlessly (the outcry when Hal Jordan was replaced by Kyle Rayner) but that the legacy of the mask is just as important as who is wearing said mask.
Beyond individual heroes and their legacies, you have entire generations ready to step up and take on the yolk of responsibility from the older generation. One of the most disliked aspects of the Nu52 was the complete removal of the Teen Titans from the prehistory of the launch, but still acting as if they had indeed existed at the same time. Batman was active for years before the events of the Nu52 started, he had three Robins and built a network of allies, but those Robins were never part of a Teen Titans, never able to work with younger heroes and hone their skills together, but when characters that were part of the Titans pre-Nu52 got together they usually acted as if they had worked together for years. This odd personality quirk is a building block for the conflict in Rebirth, that the heroes are connected but something has intentionally interfered with their memories or their reality. The Titans were the next generation of heroes, and they were the ones helping what would be considered the third or fourth (depending on when you start counting.) generation of heroes come into their own, again gathered together in teams like Young Justice or the Reavers. You always had a chain of support from the elders all the way down to the rookies. The Nu52 removed that chain, beyond Batman the heroes of the Nu52 were all new, all rookies, and you had a case of only one teacher in a room packed with children. Rebirth looks to begin righting that confusion using Wally as a catalyst, the only one that remembers the pre-Flashpoint universe, and while not all the changes of the Nu52 will be swept away, hopefully enough of them are so that the feeling of generational respect is returned.
In the end, Rebirth is more like an unboot with a return of ideas from before the Nu52 and a return of a respect for the legacy. The surprise involvement by (Spoilerspoilerspoilers) Doctor Manhattan and Ozymadias brings a very moral grey area to this idea of the Nu52 as an experiment, almost meta in its reveal that the darkness and the missing history were intentional. We can only wait to see how Rebirth continues and if it will keep on playing with that idea.