Examining the Rebellion Prior to Star Wars: A New Hope
The default Star Wars RPG campaign tends to be a Rebellion against the Evil Galactic Empire. It's actually been a while since I ran such a game - my previous Star Wars campaign was focused on smugglers and bounty hunter type characters who, while not fans of the Empire, were not outright Rebels. During the WotC era I found the Knights of the Old Republic campaign to a lot of fun. Taking place some 4,000 years before the Star Wars films, it dealt with the Republic fighting threats like the Mandalorians and the Sith Empire. Dark Horse Comics first explored this era in their Tales of the Jedi comics, followed by BioWare's Knights of the Old Republic video games. Dark Horse later did their own Knights of the Old Republic comic - one of the best Star Wars comics written, dealing with the renegade padawan Zayne Carrick and his motley group of companions. Though no longer a part of the official Star Wars canon, the tales remain fantastic and are now available from Marvel Comics (at least digitally - I don't believe they've yet done a physical reprint).
What of the Rebellion? West End Games focused the Rebellion on the period between Episodes IV and VI, an understandable decision given the period prior to Episode IV was fairly vague until the late 1990s with The Phantom Menace. Newer material has fleshed that era out quite a bit, given a number of options for a game against the Empire.
From the very start there was resistance to the Empire. In deleted scenes from Revenge of the Sith and in its novelization, the Delegation of 2,000, during the end of the Clone Wars, petitioned Palpatine to release his emergency powers. It didn't go so well.
The novel Catalyst, set during the Clone Wars and in the years shortly after it, deals with the initial construction of the Death Star and explores Galen Erso's role in it. It serves as a prelude to the film Rogue One. We find Imperial officers like Tarkin dealing with Separatist holdouts. Saw Gerrera, seen in Rogue One, Clone Wars, and shortly in Rebels, led partisans against the Empire in the Salient System, 19 years before the Battle of Yavin.
The novel Lords of the Sith shows a far larger insurgency on Ryloth, led by Cham Syndulla, father of Hera from Rebels. While the novel takes place 14 years before the Battle of Yavin, the insurgency itself was well established by that point and would go on for years after, being featured in Rebels as well.
The animated series Rebels began 5 years before the Battle of Yavin and deals with the crew of the Ghost as they fight against the Empire - first as an isolated cell and then as part of a larger cell. The novel A New Dawn dealt with the Rebel Hera first meeting the former padawan Kanan, taking place 11 years before the Battle of Yavin. The previews for the second half of season 3 suggest we'll soon be seeing the various cells being brought together by Mon Mothma - by the time of Rogue One, taking place just before A New Hope, the Rebel Alliance fully exists, though one that relies on a difficult to achieve consensus prior to performing major operations.
As far as the timeframe for our game goes, its something I'll be discussing with my players. I suspect the game will be most satisfying if the Rebellion has at least some maturity - the earliest I could see doing the game would be around the time of the first season of Rebels. This allows the characters to have a large role in the formation of the Rebel Alliance and have their own victories against the Empire. The other possible starting points I could see would be shortly before Rogue One, with the Alliance in full existence or shortly after A New Hope, an event which likely helps the Alliance greatly with recruitment (and probably leads to some dangerous optimism that the Empire quicky quashes - one could picture a Tienanmen Square like scene).
I'll note Rebels is a great source of inspiration for a Star Wars game - it's a very well done show. It's seen a number of elements that had been relegated to the Legends continuity brought back into the main canon, sometimes in different form. For example, Rebels introduced the B-wing fighter as being built at Shantipole and saw the return of Grand Admiral Thrawn.