Just saying before I begin. This is all pure speculation, just ideas I have about the upcoming film based on my thoughts and views of the Sequel Trilogy so far.
There should be a time jump of at least 5 years.
Opening crawl – we find out that Leia has died. Either succumbing to her injuries in TLJ or assassinated by Kylo’s forces. Possibly even the Knights of Ren.
Rose, Poe and Finn are leaders in the new Rebellion.
At some point after TLJ, Leia negotiated an alliance with the Chiss and (maybe) Mandolrians.
Rey has started training a small group of future Jedi, learning from the Ahch-To books. They are more balanced in their use of the Force rather than the ‘Light’ side as shown in the prequels.
A battle goes wrong for the Rebels and Finn goes to Rey and her Jedi for help.
Rey and her Jedi will take on Kylo and the Knights of Ren.
Rey and Kylo will be evenly matched. So much so that neither can actually win in a fight.
Finn is severely injured or killed in an attempt to subdue Kylo long enough for Rey to capture him.
Ren is imprisoned, and will stay imprisoned until the next Trilogy.
The Rebel army enlists more soldiers and ships to their cause and they take on Admiral Hux and his fleet. The First Order are caught off guard when the Chiss and Mandalorian armies arrive and are overrun by the opposing forces.
The First Order flagship is destroyed, killing Hux and leaving the rest of the fleet in disarray as they are now outnumbered.
Phasma has survived, but is horribly scarred after her last encounter with Finn. She leads ground troops against the Rebel army led by Rose. Rose and Phasma have their face off with Phasma finally biting the dust.
Rey and Poe (maybe Finn if he survives) manage to unite the galaxy under the Galactic Alliance.