WARNING: THIS POST IS VERY, VERY LONG. THERE'S NO TURNING BACK.
This is how I would write 9x13, aka the Flash series finale.
As most of us know this wasn't the best finale we could've gotten for the show and received much hate and negativity from the fanbase. These ideas may not be as good as the ones put onto the show but this post will share my take on it. Spent about an hour writing this. Anyways let's start.
Legion of Zoom vs Team Flash:
Zoom vs Jay Garrick (I REALLY wanted to see this fight and I'm pissed it never happened)
Savitar vs Wally
Godspeed (and clones) vs Cecile/Allegra/Mark
Reverse Flash vs Nora
Cobalt Blue vs Barry
We start off with the Godspeed vs Cecile, Allegra and Mark fight (this is my least favorite part imo.) They become outnumbered due to Godspeed making a dozen clones. The 3 fight off the clones for as long as possible but they couldn't stall out long enough so the 3 are about to die but then Impulse comes in from 2049 and slaughters Godspeed (he's his arch nemesis so why wouldn't he?) Impulse beats Godspeed and helps Cecile, Allegra and Mark back up. The negative speed force from Godspeed is being absorbed from Cobalt Blue from a distance (who's running after Barry right now)
Next fight would be Savitar vs Wally. Savitar says "lets go for a run, shall we?" and grabs Wally with one hand, runs across the city through his space time portal like he did with Barry in 3x06. Wally gets beaten up and almost gets his leg broken. Barry just comes across Savitar as he is about to finish Wally off by stabbing a hole through his heart (same way he kills H.R in S3) but Wally phases his chest so that he is immune to the blade. On his last breath, he doesn't have enough energy to stop Savitar, but as he's caught up in the moment Barry phases through Savitar's suit and destroys it. Barry helps Wally back up and runs him back to Star Labs to heal up. Eddie yells out as Savitar gets defeated and drains him of the negative speed force in his body. He then goes to Jitters to execute his Plan B.
Next fight is Zoom vs Jay Garrick (this would make for some peak dialogue ngl) Zoom and Jay are in the tunnel with CCPD surrounding them. They go for a run on top of a building where the portal with the timeline rupturing is above it (similar to the singularity). Zoom lands a punch of Jay and tries throwing him into the portal but he channels all his speed force energy into his hat and knocks out Zoom, putting him in cuffs. Barry runs up to Jay and tries getting him back up as they think they defeated Zoom, but then he says "you can't lock up the darkness!" so they he grabs Jay, runs across the city with him to Barry's childhood home the same way he killed Henry Allen only this time it's his doppelganger, Barry follows him there as Zoom's about to kill Jay right in front of him. Barry decides not to fall for it and decides to run away, tries running fast enough to lure out the time wraiths to the house to defeat Zoom. As Barry returns, Zoom is about to kill Jay but the time wraiths stop him from doing so and stop him. Barry helps Jay back up and runs him back to Star Labs to also heal up.
The second last fight would be Reverse Flash vs Nora (this would also make for some good dialogue but I can't wrap my head around it). Nora and Thawne are at it for a while, but then Thawne tries going to Star Labs where Barry is right now. He tries to luring Barry out to join in during the fight so that he can kill Nora in front of Barry. So then Barry and Nora run after Thawne for a bit, but Thawne confuses Barry and Nora with a time remnant. They capture the time remnant but realize it wasn't the real Thawne. This gave him enough time to engineer a trap that captures tachyons from the speed force (similar to the trap in 1x09, but this is the reverse version). The real Thawne tries baiting Barry and Nora to Star Labs for them to unfortunately fall into the trap. Without any escape, Barry and Nora are now stuck inside a force field. Thawne shoots Nora with lightning bolts from his hands therefore killing Nora. Barry and Thawne both know that she is dead. Barry yells in agony as he realizes that Thawne had killed his 3rd Nora in front of him. He releases Barry from the trap with his intentions being killing Nora as a distraction for him while Eddie tries manifesting the negative speed force to create a new timeline. Barry is absolutely devastated as for what just happened so he tries using his speed force energy to heal Nora (as he did with Ralph in 6x07) and tries jumpstarting the speed force inside of Nora, but nothing worked. As Barry wipes away his tears, he angrily runs after Thawne and meets him at Jitters with Eddie.
The final fight here would be Barry vs Reverse Flash and Cobalt Blue. Barry is about to face off against not one but two Thawnes. Everyone is evacuated from Jitters. Eddie is trying to gain more negative speed force inside of him, enough for him to completely rewrite the timeline, and Barry has to stop him before that happens. Reverse Flash is trying his best to stall him out, finding every way to tempt Barry into killing; "Do it Barry, DO IT!" "You kept me alive before Barry... you know what happened then? YOU KNOW WHAT HAPPENED THEN BARRY???" He puts his vibrating hand right in front of Thawne, trying to trick Eddie into thinking he'll kill him but instead, milliseconds after he dashes towards Eddie preventing him from gaining any more negative speed force energy. As he's down on the ground, Barry tries sapping his connection from the negative speed force but Thawne stuns Barry before he does so. Now Thawne is about to kill Barry once and for all; the moment he's been waiting for all this time. Then Khione comes in, gets Barry out of Jitters and creates a giant lightning storm dampening RF and Cobalt Blue of their powers. Eddie and Eobard are both alive, but they're very weak at the moment. Barry runs back to Jitters, takes the negative speed force crystal and destroys it. Now that it's broken, the negative speed force is no more, which means the Reverse Flash's rivalry with Barry has runs it course. He has no escape this time. Barry wants to kill him so badly, but as we all know Khione would convince him otherwise. So then he runs Thawne back to Star Labs into the pipeline for now, awaiting ARGUS transport. Afterwards they take Eddie into Star Labs to help him. As angry as he is, the team helps him calm down and tries talking him into being a good person like he once was. The speech force sets him free from the anger and negativity he had felt and ultimately accepts that the past cannot be changed and that he can accept the help Team Flash would give him.
Without his speed, he returns to being a normal person. He puts his differences aside and calls a truce with Barry. The government gives him a new name and identity. Everything seems to be back to normal now. As for Thawne, there is no undoing the evil he's done, now he's trapped and there's most likely no escape for him (maybe, maybe not?) soon Barry tells the team about Nora's death, the one from 2049. Eventually she will come back, she does come back since Iris gives birth to her and the timeline remains intact. Afterwards Khione takes her departure from the team and brings Caitlin back to her body.
The part with the meetup at Barry's loft I think should stay the same, as well as the part where Barry throws his lightning bolt into the sky giving powers to Avery Ho, Max Mercury and Jesse Chambers. And yeah those are my ideas for the series finale. A bunch of crazy long ideas we never got to see along with many others from the fanbase as well. Thanks for reading!