It was confirmed in 2014 that the Star Wars Expanded Universe is dead, with The Force Awakens forging its own new canon story and universe whilst disregarding the cult stories which have been running since 1991.
The Star Wars EU was home to the continuation of the Star Wars plot after Return of the Jedi left cinemas, allowing fans to discover new adventures and characters such as Mara Jade or the Yuuzhan Vong. All of these classic aspects of the Star Wars saga were beloved by fans for many years, precisely because it was the only continuation of the original saga we were left to believe existed, as the prequels instead came into being and the mystery of Skywalker's future was left by the pyres on Endor.
A message to the fans and some small form of consoling was brought about by one of the publisher's of the EU books, Del Rey's publisher Shelly Shapiro, explaining that.. “Well first of all, we don’t want to just disappear stuff that everybody read and loved – including myself. Legends are things that are often told over generations so they’re not… they change constantly with the telling, so you can’t actually attribute an author to any particular one. Often it wasn’t someone who was actually there. You can go back to any of the legends… they’re pretty sure there was a ‘King Arthur,’ but most of the stories probably did not happen. But that doesn’t mean there aren’t kernels of truth in it.". What we didn't realise however, is how that applies directly to Star Wars: The Force Awakens...
SPOILERS BELOW. DON'T READ DOWN IF YOU HAVEN'T SEEN THE FORCE AWAKENS.
SERIOUSLY. DON'T SCROLL DOWN IF YOU HAVEN'T SEEN THE FORCE AWAKENS
*Spoilers start now*
Disney has confirmed that the EU is "dead", at least as canon, however that doesn't stop JJ Abrams and all authors/scriptwriters of the future from cherry picking the vast historical Star Wars resource available to them. There are characters such as General Thrawn or story lines like the death of Chewbacca that resounded so loudly with fans over the years, that it is almost a fallacy to not replicate or imitate these elements in future canon that audiences have enjoyed in the past. Indeed the EU isn't dead at all, but is now part of a Legends book line-up and collection online, a renaming by Disney to pay respect to the decades of work that went towards the saga, but giving them a breeding ground and breathing room of their own right. The EU is now a parallel canon universe recognised by Disney, but is a separate continuation of the Star Wars story in book form, separate from the 'canon' film continuation.
Luke Skywalker attempts to rebuild the Jedi Order
The Force Awakens brought to light that Luke Skywalker tried to rebuild the Jedi Order in the wake of Emperor Palpatine's defeat. Unfortunately this was all for nothing when one of his students, his very own nephew — Kylo Ren — turned against his Jedi Master and brutally cut down all the other Jedi Apprentices (or so audiences know at the moment), but whether this was brought on alongside the other Knights of Ren (or even if the other apprentices are now part of the Knights) is not yet revealed. This resurgent of the Jedi Order after Star Wars 6 was quick lived and bloody in its demise, sending Skywalker into seclusion for years whilst searching for the first Jedi Temple.
This is a direct mirror of the EU, in a trilogy of Jedi Academy books, where the 'New Jedi Order' never actually fell apart, but Luke still fell foul to betrayal by one of his students (and family) whilst trying to bring peace back to the galaxy. Luke even left into exile at one point for not stopping Han and Leia's son Jacen Solo from turning to the Dark Side and nearly destroying the New Republic...
Ben "Kylo Ren" Solo-Organa/Skywalker
Whilst being a bit more loose in the connections to the EU, there are still several connections from "Han Solo's EU son" to the Kylo Ren character in Force Awakens.
Ben Solo, as we found out in The Force Awakens, turned against his family and friends in the brief history before the film began, betraying everything he knows and turning to the dark side away from Jedi Master Luke and under the control of Supreme Leader Snoke. Even Ben's own name is derived from Luke Skywalker's own son's name in the EU (This is more likely a calling to his own mentor Obi-Wan, but still a nice EU nod...).
Han and Leia meanwhile had a more joyful post-ROTJ relationship in the EU than their film counterparts (estranged for who knows how long?). Their flirting and marriage led to three children: twins Jacen and Jaina, and their youngest, Anakin. Jacen turned to the Dark Side whilst assuming command of the New Republic, but in secret hiding the fact he was now a Jedi. Jacen was eventually cut down by his own sister Jaina, this could be a potential contrast to Rey (who's parentage is as yet unknown), however not before cutting down his own mother fan-favourite Mara Jade. Could we see similar happen in the canon universe? The reason Kylo Ren is so feared and shunned by his own family in the EU is because he murdered Luke's wife before escaping to Snoke?
The Sun Crusher
The Force Awakens features a riff on the Death Star, bringing Starkiller Base into the fray as the latest superweapon to pose threat to The Resistance (and a nice name nod to Skywalker's original scripted name). The sheer destructive power of Starkiller Base is exhibited in The Force Awakens as we see it wiping out seemingly all of the New Republic and several planets, whilst absorbing the power of a nearby star, to enable the First Order to destroy any opposition.
The EU superweapon on the other hand is pretty similar to Starkiller Base, with it also having the ability to wipe out whole systems. The only difference of the two being that the single-pilot craft Sun Crusher can get close to star's with its impenetrable armour, using torpedoes to explode the cores of stars and trigger destructive supernovas.
Darth Plagueis and The Muun
There has been a lot of rumbling amongst the fan base recently that Supreme Leader Snoke is actually a familiar figure to most fans...
Emperor Palpatine was established in the prequels to have trained under a Muun Sith Lord called Darth Plagueis, a Sith Lord who had unlocked the power to eternal life and the ability to defeat death itself through the Force. After discussing this with Anakin amongst a theatre act performed by dancing holograms and swooping ghost jellyfish, Plagueis is never heard from again...
Could it be though that the massive hologram introduced in The Force Awakens is merely a front for the tall thin form of a Muun Sith Lord? It's not much of a stretch to imagine that Muun pioneered a new age of Sith development, where the rule of 1 master and 1 apprentice was other thrown and he operated in the shadows whilst Palpatine was the 'face' of the Sith for many years, only coming forth from the shadows when Vader and Palpatine finally died in ROTJ. There is a whole book in the EU discussing the relationship between Palpatine and Plagueis, however it's not much of a stretch to imagine this either being brought into film canon, or reutilised to show that some Force trickery lead Plagueis to operate from the shadows for 50+ years (with his eternal life...) and eventually become Supreme Leader Andy Serkis... sorry, Snoke.