Are there any other arcs or episodes relevant to Rebels that I should catch before Netflix pulls them?
If "everything from season 3 onwards" is too much of an ask then in no particular order: -
('Overlords', 'Altar of Mortis' & 'Ghosts of Mortis'): The Mortis arc in season 3 is an absolute *must* for any Star Wars fan. You may not understand half of what's going on (hint: there's a *lot* of metaphor layered in there) but this is where George Lucas opened the lid on some of the deeper cosmic mysteries of the force with some seriously mythical stuff and it's bonkers (in a good way!) .
('The Citadel', 'Counter Attack', 'Citadel Rescue', 'Padawan Lost' & 'Wookiee Hunt'): For a broader appreciation of Ahsoka's character arc (and an early appearance of Tarkin), you should definitely check out the last two arcs of season 3 as well as...
('Heroes on Both Sides' & 'A Friend In Need'): Since those also overlap with the Saw Gurrera/Onderon arc, the Mandalore arc and I think will play a significant part in the upcoming Sabine episodes on 'Rebels' (also, if you haven't seen the Mandalore episodes, go watch *all* of them!)
('The Gathering', 'A Test of Strength', 'Bound for Rescue' & 'A Necessary Bond'): Aside from advancing Ahsoka's character, the padawan arc is important in a number of ways: it gives some deeper lore on the Jedi and kyber crystals, adds more context to Luke's cave vision in ESB (plus all the goings on in the Lothal temple) and despite featuring another appearance from Hondo it's actually guest star David Tennant who steals the show!
('Landing at Point Rain', 'Weapons Factory', 'Legacy of Terror' & 'Brain Invaders'): Also related is the Geonosis arc as it sets up Ahsoka's friendship with Barris, furthers her relationship with Anakin and as one might guess, gives a little more context to the Geonosis episodes on 'Rebels'.
('Darkness on Umbara', 'The General', Plan of Dissent' & 'Carnage of Krell'): The Umbara arc is a strong one for Rex, gives a little insight into the downfall of the Jedi and has some of the best warfare action in the whole show (not surprisingly considering the guest director was
the editor on Apocalypse Now.)
('The Unknown', 'Conspiracy', 'Fugitive', 'Orders', 'The Lost One', 'Voices', 'Destiny' & 'Sacrifice'): Season 6 has some *hugely* important episodes that relates to the mystery of Sifo-Dias, Order 66, Qui-Gon, Dagobah, force ghosts, and a special guest appearance from Mark Hamill.
('Deception', 'Friends and Enemies', 'The Box' & 'Crisis on Naboo'.): I don't think it plays into 'Rebels' but this is a very good arc about Kenobi infiltrating the underworld that quite surprisingly circles back to Anakin's gradual corruption by Palpatine
('Nightsisters', 'Monster', 'Witches of the Mist' & 'Bounty'): If you just watched the Maul episodes in which he appeared then you missed out on these lead up episodes which mostly focuses on Ventress but also gives some insight into Maul's heritage, our first look at Dathomir & the Nightsisters and turned Asajj herself from a two dimensional thug into a much more interesting and fleshed out character.
('Holocron Heist', 'Cargo of Doom' & 'Children of the Force'): The Cad Bane arc from season 2 has a surprisingly amount of relevance to the origin of the Inquisitors, a certain someone's cameo in 'Rogue One' and the season 1 finale of 'Rebels'.
Honourable mentions: -
('Bounty Hunters'): is basically a 22min remake of 'The Seven Samurai' and showcases some recurring bounty hunter/mercenary characters who show up elsewhere (including some the the episodes mentioned above.)
('Lightsaber Lost'): By no means vital, but still a solid Ahsoka episode.
('Death Trap', 'R2 Come Home' & 'Lethal Trackdown'): An arc dedicated to Boba Fett's vendetta against Mace Windu and features Aurra Sing in a memorable guest role.
('Assassin'): Another Aurra Sing appearance, this time with Ahsoka & Padme on Alderaan (and also Mon Mothma is there.)
('Clone Cadets', 'Rookies' & 'ARC Troopers'): These are pretty much the backstory for Fives who, aside from Rex & Cody is probably the most important clone trooper in the series as he has a significant role in the Citadel, Umbara & Order 66 arcs.
('Kidnapped' 'Slaves of the Republic' 'Escape from Kadavo'): Being the only EU story that George Lucas *ever* directly adapted into canon should be reason enough to watch this arc, but it also happens to offer greater insight into Anakin's mindset, show's Kenobi gaining a new perspective on them and gives Ahsoka quite a bit to do as well. Also it may or may not imply that the whole "Jabba's Sail barge" plan was R2's idea...
