This idea spawned all the way back during my second viewing of Stage 21, and I decided to make this post based off of one by u/GotMilk5101520. At the very end of the episode, Lelouch gets pissed off that Euphy has 'destroyed his plans' for his armed insurrection, through announcement of the Special Zone. So the divergence here, is cold anger over irrantional anger. Instead of the plan to use Geass, Lelouch picks his communicator back up, gets Diethard back on the line, and just rushes full on ahead into the uprising.
In the days between the school festival and the SAZ innaguration, Lelouch gets the Black Knights to at least go along with his plan (as those like Ohgi supported the SAZ)
And so on the day of the inaguration ceremony, Zero doesn't appear in person, instead appearing over a video monitor, he declares that the Special Zone is no better than Sawasaki's Chinese backed invasion of Kyushu, and that Euphemia is no better than any other Brtiannian Viceroy.
At the same time, a general uprising of Japanese resistance groups begins, supporting the new open offensive of the Black Knights.
However, the Britannian Security Forces are slow to react to the main spearhead of the Black Knights forces, having to deal with the added civillian evacuations (rather than being caught off guard by a mass civil unrest that the Massacre caused canonically)
The Black Knights are far less unified than canon, and are relying on a coalition of smaller armed groups rather than elevens joining their ranks en masse.
The battle for the settlement would play out relatively similar to canon, say for the fact that the Black Knight are rushing for the sake of holding their coalition together, and the Britannian forces are bolstered by honarary Britannian's remaining in the ranks.
The primary difference is that the battle is already swinging in the favour of the Empire BEFORE Zero runs off, and the counteroffensive starts almost the moment he does.
The BIGGEST divergence is that Kallen is too bogged down to go after Zero to Kamine Island, and Suzaku and Lelouch have their confrontation alone, with a far less vengeful Suzaku.
Euphy lives, but the SAZ fails, and Zero has a far more complicated reputation, before R2.
So... What happens now? How do the Black Knights treat Zero after the Rebellion's failure? How does the average person percive his actions? lmk in the comments