Side stories are always hard to pull off, especially in something like Star Wars, where one of the main features of the primary material is how grand and sweeping its story is. A New Hope opens with Rebel starships having just won their first victory, proceeds through the Empire discovering the main rebel base and the rebellion confirming the existence of the Death Star, and ends with the rebels defending their base and Luke hinting at the rebirth of the Jedi Order. A side story in the same era can't do any of those things without stepping on ANH's toes, but it also can't have nothing to do with them or it feels out of place... And Rebels successfully resolves that paradox.
Kanan and Ezra are the perfect way to have Jedi in a Rebellion-era story without it being a story about the rebirth of the Jedi Order... Or the failure to do that. Kanan and Ezra are both scarred orphans; Kanan's not even sure he can teach Ezra to be a Jedi, much less that he wants to. His confrontations with Sith Lords end in disaster. And Ezra winds up being a different kind of Force foundling from Luke. No possible way for this boy to restore the Jedi Order. He's more what the Jedi feared, what led them to take children from their homes for training. Ezra isn't just impulsive and ungovernable, he's wild and primal. He's got a special Force rapport with beasts, and while it often scares him, he frequently draws on it all the same.
Hera's cell's trials give a basis for the action of ANH. At its best, Phoenix Squadron only ever manages to establish a tenuous base, with a handful of ships and a small fighter squadron. Their A-Wings are tolerable recon and raiders, but practically helpless against mainline Imperial Fleet ships. The Ghost, a single converted freighter, is their best combat asset. Fuel is short, safe travel nonexistent, any kind of success met with immense reprisal... And joining up with Dondonna and Mon Mothma doesn't make things better, as their galaxy-spanning agenda is a poor fit for the direct action and local concerns of Phoenix Squadron. The show wisely holds out on X-Wings until the very end, and seeing Hera hold her own against a cutting-edge Imperial prototype in one is cathartic and really sells the capabilities of the Alliance's signature hardware. Yavin base is bigger than anything Phoenix Squadron ever pulled together, so it's discovery and the equally outsized firepower the Empire musters against it fits as an escalation from the "early rebellion" stories here.
Rebels weaves its themes tightly throughout its run. The consequences of war are inescapable; even sixteen years later, the Clone Wars loom large in everyone's memories. The Rebels don't always win, and even when they do it's seldom without a cost. Family would seem to be one of the most trite Star Wars themes, but it works here. Apart from the tenuous "found family" of the Ghost's crew there isn't a single intact family in the show, and their dysfunction is a frequent plot driver. And even among the Ghost's crew, Kanan and Hera's exact feelings are left ambiguous until almost the very end. Sabine's disaster of a Mandalorian family is particularly delightful, both in its own right and going against the stoic loner image of Boba Fett.
Rebels, of course, doesn't only relate to the original trilogy, but to its contemporaries. Rogue One gets some smooth tie ins; we see some V-Wings and troop gunships, and Saw Gererra is featured in several episodes, including an actual explanation of who he is that Rogue One itself mostly forgot. In fact, his appearance is the closest the show ever gets to the Death Star, and that isn't especially close. Saw's obsession with finding the empire's super-weapon is shown as something of an impractical quest, one that sets him against insurmountable opposition and means he passes on opportunities for more immediate good.
Rebels' relation to Solo is... More fraught. Maul is an antagonist throughout the second and third seasons, ultimately falling to Obi-wan on Tatooine in mid season 3, aired in late 2016. By the time Solo hit theaters in 2018, with the twist finale reveal of Maul's survival as a galactic crime lord, he's not only long since abandoned that role, he's been dead for nearly two years. Disney had made a big deal about how the they wanted a single unified canon, without the confusing levels or rewrites of the old EU... So to me, their handling of Maul really shows how little actual planning they've done.
Finally, a number of the late series elements suggested a strange relation to me. In the early 90s, just as the EU was kicking off, a trilogy of serial-numbers-filed-off Star Wars-alikes called the "Mageworlds" was published. They do a number of fun things with the formula and concepts, but saw little commercial success. One of their more original concepts was that the antagonist Mages were capable of traveling through hyperspace without a spaceship - that hyperspace was, in fact, a mystical dimension linking many times and places that hyperdrives transited in the most limited, pedestrian manner. Rebels, oddly, uses this exact concept in its final season, with a story arc involving "the world between worlds" and Force-connected beasts that explicitly "walk" paths through hyperspace. I'd be shocked if any of the writers intentionally drew from such an obscure relation, but... The parallels in imagery and narrative are strong?
