One of the most common rumors that has been going around is that General Leia ends up in a coma and Laura Dern’s character takes over as leader of the Resistance. I can confirm that this happens but I am able to elaborate a little bit on how it happens.
The opening scene in the film is set at a memorial service for the people who died in the Hosnian Prime attack and the Resistance pilots and soldiers who died on Starkiller Base in The Force Awakens. From my understanding the scene takes place on Corellia. General Flux described it as a high-tech, industrial style city on the waterfront. It just reminds me of Corellia but I could be wrong as it may be a brand new world created by Rian Johnson and co.
During the funeral procession a First Order assassin attempts to kill General Leia. The assassin is described to me as a white male wearing a black cloak but when he attempts to make his escape you can see he is wearing a gray military uniform with black armored chest and shoulder pads that go up to the man’s head and armored gauntlets with a faint asterisk looking symbol on the shoulders when the cloak was flowing behind him as he ran. I did some digging and it seems to be vaguely similar to The Inquistor from Rebels but with a First Order symbol rather than the Galactic Empire symbol. The man doesn’t use a lightsaber I was told but rather a blaster of some sort and shoots Leia. The assassin is caught by local security but kills himself using the Star Wars version of a cyanide capsule.
Initially I thought the assassin was a Knight of Ren, but I don’t recall any of the Knights wearing the First Order’s insignia so that leads me to think an expendable warrior like a Stormtrooper but trained for assassinations rather than open warfare.
Finally, I’m told Leia is able to keep herself from dying by using the Force, long enough for medical crew to get to her to a hospital or whatever they call it in Star Wars. Leia loses consciousness and is in this state for the majority of the film. General Flux has confirmed to me that she does wake up near the end of the movie thanks to Luke using some type of Force technique to pull her out of the coma.