Luke Skywalker is one of the greatest Jedi in the entire Star Wars franchise, whose actions brought about the fall of the Galactic Empire and restored peace to the galaxy. His exploits as a Jedi Master have recently come to the spotlight through his appearances in several recent Disney+ series.
Although he certainly succeeded in his endeavors as a Jedi, Luke Skywalker had to do so without the guidance that other Jedi would have had during the age of the Republic. There are many Jedi Masters whom Luke never had the chance to study under, but could have learned valuable lessons from.
Kit Fisto - Coping With Loss
Kit Fisto was a master lightsaber wielder and a member of the Jedi High Council throughout the Clone Wars conflict. Though he eventually met his end at the hands of Sheev Palpatine himself, Master Fisto was still one of the most powerful Jedi Masters in the final days of the Republic.
Kit Fisto suffered the loss of his former Padawan learner, Nahdar Vebb, after a conflict with the droid General Grievous during the Clone Wars. However, in true Jedi fashion, Master Fisto was able to move past this loss--something that Luke Skywalker seemed averse to. Fisto would have been able to teach Luke a better way to cope with grief than what was seen in The Last Jedi.
Yaddle - Sacrifice
Master Yaddle makes only a single live-action appearance in the Star Wars franchise when she is shown as a council member during The Phantom Menace. She is one of the few known members of her species, with other notable examples including Master Yoda and Grogu.
Master Yaddle is reported to have died before the events of Attack of the Clones when she sacrificed herself to save the life of the young Anakin Skywalker. This lesson of sacrifice is something that Luke would eventually learn prior to his own death, but that would have been bolstered by the teachings of a wise older Jedi that already intimately understood such things.
Saesee Tiin - Increased Force Abilities
Saesee Tiin was a Jedi Master at the time of Order 66 who had an exceptional command over the Force, even to the point of being able to pilot a ship purely through his telepathic abilities. He died shortly before the execution of Order 66 when he accompanied Mace Windu to arrest Chancellor Palpatine after his revelation as a Sith Lord.
Master Tiin may not have been depicted using his Force abilities all too often, but he certainly could have taught them to a curious Luke Skywalker. Luke, though strong in the Force on his own, would have benefitted from a Jedi so talented in this area, thereby becoming a more powerful Jedi as a result of his teachings.
Quinlan Vos - Breaking The Rules For The Right Reason
Quinlan Vos was considered to be a "maverick" among the Jedi Order, whose methods were often viewed with distaste by his superiors. Master Vos was revealed to have survived the Great Jedi Purge when he recently got a shout-out in an episode of Obi-Wan Kenobi, indicating that he might have a future in the franchise yet.
While a meeting between Quinlan Vos and Luke Skywalker is not entirely off the table, it seems unlikely given the chronology of the franchise. Nevertheless, Master Vos had a tendency to break the rules of the Jedi when it proved necessary, exchanging rigid doctrine for flexible philosophies that maximized his heroism, all of which Luke himself very nearly failed to learn in his time as a Jedi.
Ki-Adi-Mundi - Wisdom In Wartime
Master Ki-Adi-Mundi was a member of the Jedi High Council throughout the events of the prequel trilogy. A Cerean Force-wielder with a lofty position among the Order, Ki-Adi-Mundi eventually met his untimely end at the hands of his own clone troopers, who shot him to death in the midst of the Third Battle of Mygeeto.
Ki-Adi-Mundi would have been a terrific teacher for Luke Skywalker thanks to his many years of experience acting as a Jedi Knight in the midst of galactic war. Though Luke sat out most of the Resistance-First Order conflict, the musings of a war hero might have encouraged him to join the Resistance effort sooner.
Luminara Unduli - Coping With A Fallen Apprentice
Luminara Unduli was a Jedi Master just before the end of the Clone Wars who trained Barriss Offee, the infamous traitor to the Republic and the Order itself. She is killed sometime after the Battle of Kashyyyk, only for the Inquisitors to later use her remains as a beacon to lure surviving Jedi into a trap.
Luminara and Luke both share a dismal similarity in that they each trained a Padawan that would eventually turn on the Jedi and murder their fellow students. Unlike Luke, Luminara managed to move past this failure, learning from her mistakes rather than retreating into anonymity.
Plo Koon - Learning To Let Go
Plo Koon was a powerful member of the Jedi Council in the days leading up to the execution of Order 66. As the master who brought Ahsoka Tano to the Jedi Order in the first place, Master Koon has an important place in Star Wars canon until his death on Cato Neimoidia.
Plo Koon was an instrumental advisor to Luke's father during the Clone Wars whenever it came time to let loved ones carry on a fight on their own. He understood when it was necessary to allow someone to fight their own battles, a lesson that the Skywalker family, including Luke himself, never seemed to fully digest.
Mace Windu - A Healthy Respect Of The Dark Side
Mace Windu was one of the most vocal members of the Jedi High Council throughout the Clone Wars. Among the most powerful members of the Order, Master Windu was not always as resistant to the Dark Side as one would expect from a Jedi Master, practicing a lightsaber form that indicated that he drew on both Light Side and Dark Side energies to wield.
Though Master Windu's mind was not always as balanced as his command over the Force, he would have made a formative teacher for the young Skywalker. After his father's struggle with the Dark Side, Luke could have used the voice of a Jedi who had learned to balance such darkness to assure him that he would not necessarily fall into his father's footsteps.
Qui-Gon Jinn - Doctrinal Flexibility
Qui-Gon Jinn was the Jedi Master who trained Obi-Wan Kenobi and first discovered Anakin Skywalker. Resolute and determined to do what he saw fit, Qui-Gon often disobeyed the wishes of the council, a reputation that kept him from earning a seat among their ranks. Qui-Gon is eventually killed by the Sith apprentice Darth Maul during the Battle of Naboo.
Qui-Gon knew when it was time to obey the tenets of the Jedi and when to break their doctrines in order to serve the greater good. This is a lesson that failed to present itself to Luke Skywalker, who spent years combing through the ancient texts of the Jedi when he would have been put to better use honing his Force skills as he saw fit. Ultimately, an apprenticeship under Qui-Gon Jinn might have helped Luke along this path in a healthier way.
Anakin Skywalker - Balance Of Moral Attachments
Anakin Skywalker is the Chosen One spoken of by the Prophecy and one of the most powerful Force wielders to ever exist. Brought to the Jedi Order at a far older age than most Jedi, Anakin always struggled with the Jedi doctrine, eventually leading to his betrayal of the Jedi and becoming Darth Vader.
Though Luke did have the chance to meet his father as he once was before he died, Anakin did not have much time to impart any knowledge to his son. Nevertheless, had the chance arisen, he would have no doubt encouraged Luke to maintain his relationships with his loved ones, but to do so with balance, knowing that a Jedi must be capable of suffering great loss should such an event arise. Thereby, Anakin would keep his son from making the same mistakes that he once did.