Star Wars has always been full of mysteries, even before The Force Awakens came along, but thanks to the cinematic revival, fans are dying to know the answers to questions like Snoke's identity and Rey's parentage, but one of the biggest curiosities is the story of Luke Skywalker after Return of the Jedi. After his near complete absence from the marketing for Episode VII, it turned out he was barely even in the movie, only showing up at the last moment when Rey finds his island of exile on Ahch-To.
While there are a number of books that take place between the events of RotJ and TFA, detailing the fall of the Empire, the rise of the New Republic, the emergence of the First Order and stories about other characters like Han and Leia, Luke has had surprisingly little revealed. The most we've seen of him after the Battle of Endor is a short mission with Shara Bey, Poe Dameron's mother, where he tracks down one of Emperor Sheev Palpatine's secret research bases to liberate two Force-sensitive trees - saplings cut from a great tree growing on the grounds of the Jedi temple on Coruscant.
After that, Luke falls off the radar. From brief comments in the Bloodline novel and The Force Awakens, we know he takes Ben Solo on as an apprentice and tries to track down Jedi relics and rebuild the Jedi Order, but his nephew turns on him, killing his new Jedi students and taking the name Kylo Ren as he aligns himself with Supreme Leader Snoke and the First Order.
The Last Jedi will, hopefully, provide a little more detail to the unknown events between trilogies, but we already know that it will for sure explain more about what Luke has been up to and share details about his island on Ahch-To.
Thanks to the second trailer, we've already seen the first Jedi temple. The first trailer revealed a bookshelf with several ancient books, and a close look at the wall shows it's more than likely inside of a tree, then in the second trailer we see Rey approaching a large tree, which can be assumed is itself the temple. What's even more interesting is that a closer look just might reveal a deeper connection to decades worth of Star Wars imagery.
The temple is shrouded in mist, but you can faintly make out the shape of the center trunk and what appears to be several ancillary trunks, branches, or separate trees (we need a Star Wars botanist to know for sure), clearly outlining a similar structure to the Jedi temple on Coruscant. Once the connection is made, it makes a lot of sense for the Jedi temple on Coruscant to echo the appearance of the very first center of Jedi knowledge and culture. Taking that same look as the humble first temple and expanding it into a much larger temple complex as the literal pinnacle of the Jedi Order is also ironic, as that same growth in the ego of the Jedi is what would eventually lead to their downfall (nevermind the fact that the site they chose was over a former Sith shrine, which didn't help).
Another interesting detail (albeit a bit more of a stretch) is that partly due to the angle of observation, but partly due to the obstruction of the mist, this new look at the first Jedi temple also suggests a connection to a symbol that goes further back in the real-life history of Star Wars than the Jedi temple of the prequels. The Rebellion Starbird has been a symbol familiar to fans since 1977, and while there's still no official connection, it appears to have a clear inspiration drawn from the Jedi symbol seen on the shoulder of Obi-Wan Kenobi's armor in The Clone Wars animated series and on the cover of an ancient book in the first trailer for The Last Jedi.
The link between the look of the temples, the Jedi emblem, and the Rebellion Starbird is a little more tenuous, but considering the Jedi Temple made out of a tree contains a book with a symbol on its cover that bears a strong resemblance to the outline of the temple, it's definitely possible that the temple/tree is the most symbolically significant structure to ever appear in Star Wars.
While The Force Awakens clearly took most of its inspiration, especially aesthetically, from the original trilogy, the use of imagery that ties back to the Jedi temple on Coruscant will be the most overt prequel connection yet. With Episode IX bringing in Chris Terrio as a writer, we can expect this kind of prequel reference to be even more prominent, hopefully tying the prequels and the originals together in what could be the saga's ultimate conclusion.
However, considering this tree temple is far more ancient than the Jedi temple that mimics it, as are the books inside, The Last Jedi isn't only set to return the saga to prequel imagery, but to also delve even further back to an era of the Star Wars galaxy barely explored in modern canon.