When it first began in 1977, Star Wars really only had two main factions - the Rebel Alliance and the Galactic Empire. As the lore developed beyond the original trilogy, more factions were added to the galaxy's eternal struggle between light and dark, but one would go on to stand out from the crowd: the Mandalorians. This warrior culture originated as an unused concept for The Empire Strikes Back, but was quickly embraced by the expanded universe. The Mandalorians would go on to have a variety of roles across different media. They were sometimes good guys, sometimes bad guys, and sometimes a bit of both.
But the Mandalorians took on a new life when they were brought into The Clone Wars and Rebels, before getting into live action with a series fittingly titled The Mandalorian. Although the primary focus of the show a single character - Din Djarin - and his relationship with a former Jedi youngling, its story would explore the intricacies of Mandalorian culture. Several other notable Mandalorian characters would become major cast members, including Bo-Katan Kryze, a character first introduced in The Clone Wars. But for all that it has done for Mandalorian culture, there are still some interesting Mandalorian characters who have not been able to appear in The Mandalorian.
5 Canderous Ordo
Canderous Ordo could easily be described as the original Din Djarin, having experienced a very similar story. He was a veteran of the Mandalorian Wars against the Republic, and later the Jedi, which ultimately left his people scattered and divided not unlike their situation in The Mandalorian. Although he never bonded with a Jedi youngling in the same way, Canderous was left trying to get by as a mercenary before, ironically, joining his former enemies against the Sith. Fighting alongside Revan would eventually put him on a path to reuniting the fractured clans under the title "Mandalore the Preserver."
There is a pretty good reason why Canderous does not appear in The Mandalorian - he would have lived a thousand years before the events of the movies, and is presumably long dead by the time Din Djarin's quest begins. If he were to make any kind of appearance, it would more likely be in the form of something exploring his legacy. While it might be cool to encounter a descendant of Clan Ordo, an artifact associated with Canderous, or even explore his tomb, his absence is understandable. However, the impact Canderous left was a strong one.
4 Gar Saxon
Mandalorians have not always had a favorable reputation in the galaxy, and people like Gar Saxon do not help. He was known to be ruthless and power-hungry, and selling out his people for his own gain on two separate occasions. His first questionable act was supporting Maul's hostile takeover of Mandalore, making an enemy of Bo-Katan. After Maul was defeated, Saxon joined the Empire, and got the job of enforcing its rule over Mandalore. His hold was only broken when he was challenged by Sabine Wren, who was so disgusted by his actions she didn't even consider him worthy of an honorable death. He was finally killed after a failed attempt to shoot Sabine in the back, ending as a disgrace.
Although Gar Saxon's death prevents him from appearing in The Mandalorian directly, his cruelty would certainly be known to the Mandalorians, and it would be recent enough that a lot of them probably have first-hand experiences of living under his oppression. There might even be some who actually knew him personally. And even the ones who were not on Mandalore at the time would likely have heard stories. Gar Saxon would be remembered as a power-hungry tyrant and a traitor to the Mandalorians.
3 Sabine Wren
Though Star Wars has produced some memorable Mandalorian characters, few stand out quite like Sabine Wren. She had a passion for art that mixed dangerously with her proficiency in various forms of combat. She even reflected this in the way she personally customized her armor and her frequent changes in hair color. She became a dedicated rebel and played a key role in rallying the Mandalorians against the Empire. Her part in the Galactic Civil War even contributed to some major plot points in The Mandalorian. It was, after all, Sabine who recovered the darksaber from Dathomir and eventually gave it to Bo-Katan.
Although the events of Rebels do indirectly set up parts of The Mandalorian's story, Sabine herself has not yet appeared, leaving the question of her whereabouts after the original trilogy open. However, these questions might get some answers in Ahsoka, where Sabine is set to make her first live-action appearance.
2 Shae Vizla
Shae was born into a volatile period of the Mandalorians' history. Her people were officially allies of the Sith Eternal Empire, though in reality they were more like servants under the control of a puppet leader who was only acting in the interests of the Sith. Although she began her career as an agent of the Sith, and held a bitter grudge toward the Jedi after the death of her brother, Shae Vizla would be crucial to the Mandalorians breaking free from the Sith Empire's control. Despite numerous obstacles, she would eventually lead the fight against them, earning the title Mandalore the Avenger. Put simply, she is the kind of person the Mandalorians would view as one of the greatest heroes.
Like Canderous, Shae has not made a direct appearance in The Mandalorian on account of her story happening a few thousand years earlier. However, the series does feature a member of her clan. Paz Vizla has been a recurring character throughout the series, and played a big role in Season 3. They have not stated if he is a direct descendant, but the fact that the Vizla Clan has survived so long suggests Shae made a pretty big impact on their culture.
1 Ursa Wren
Ursa had the misfortune of being caught in the crossfire of the Clone Wars, initially as a member of Deathwatch alongside Bo-Katan. Towards the end of the war, she helped liberate Mandalore from Maul's control, only for it to then be taken by the Empire. Ursa quickly found herself in the difficult position of having to lead Clan Wren through Imperial occupation, a task which forced her hand in some extremely painful ways. Like having to exile her own daughter after she was branded a traitor and then having to keep up the appearance of being pro-Empire. Despite all her problems, she managed to keep Clan Wren intact and would eventually reunite with her daughter on a second mission to liberate Mandalore, a campaign which proved far more successful this time.
Ursa's whereabouts are not entirely clear in The Mandalorian, although the role she played in Rebels did help to set up its events. The series has not brought up Clan Wren, though her daughter Sabine Wren is expected to come back in Ahsoka. Sabine's continued story and her connection to Bo-Katan both seem to leave open possible avenues for her return.