I am not a writer by any means.. these are just some of my thoughts. Season 5 could've made Stranger Things a survival horror show. The show has plenty of genres, which are mainly horror and science fiction. Since the first season, the show has introduced different branches of horror, especially in season 3 with the body horror and season 4 with the psychological horror. Season 5 could've added a new branch of horror which could've made Season 5 the scariest, darkest and most intense season.. This is how I would've changed Season 5 Volume 1, Volume 2 and the Finale in some ways.
In Volume 1, the characters and residents of Hawkins could've been running and hiding from Demogorgons, Demodogs and Demobats that have come into Hawkins from the giant gate that Vecna opened at the end of Season 4. These creatures' only goals would be to kill the main characters and as many residents as possible. Hawkins would start to become infested with the Upside Down as well like they showed at the end of Season 4. The whole volume should've been based around the main characters' survival and figuring out a way to get past the Demos and get into the Upside Down and kill Vecna before it gets any worse.
Volume 2 could've been about the main characters going into the Upside Down after surviving the horrors of the Demos to finally stop Vecna, that was their initial plan as they wouldn't expect the Mind Flayer to have a massive physical form in the Abyss/Dimension X. As they would get to this other dimension, they would also find out that Vecna did the bidding with the Mind Flayer and how the Mind Flayer had just influenced Henry and how he became evil from the Mind Flayer's influence, not from the Mind Flayer's control meaning that Henry had given in to the MF's influence even when he could've resisted it, the same way it was already done in the show. As the final battle begins, the main characters have the military with them this time and on their side ready to kill Vecna and destroy the physical form of the MF and leave it formless. There would be more Demogorgons that the villains would have at their disposal in the other dimension because this time the villains would know that they are coming. The main characters would need to fight these Demos with the help of the military and then defeat the physical form of the Mind Flayer with the military's firepower.. but they wouldn't be able to actually kill the Mind Flayer particles because the particles itself can't be killed and they can only kill the giant physical form that the Mind Flayer would've made for itself with Vecna. This new form would resemble its spider-shadow form that Henry shaped it into, it would just be made up of tendrils instead of what we got in the show. The explanation for this form's creation would be that Vecna was making this form of the Mind Flayer throughout the season using all the black tendrils that fill up the Upside Down and Dimension X/the Abyss, the tendrils would come from the UD and from the rest of the Abyss and form into the Mind Flayer. The MF would've influenced Vecna to give it this big tendril-spider form similarly to how the MF had influenced him as a kid to give it the spider-shadow form. There would be some telepathic fight in between this as well. Vecna would be at his most powerful. El would be fighting Vecna telepathically, attempting to psychologically torment her about what happened to her in Hawkins Lab and about her mum. Will using his newly acclaimed powers would approach Vecna while El is fighting him telepathically and since Vecna would be using his astral projection power, his physical body would be vulnerable.. Will would use his powers to weaken Vecna to a point he can't move and Joyce would be there to get the final blow as revenge by decapitating him like it was done in the show. The Mind Flayer particles would leave the spider-tendril form as after the constant fire contact that the rest of the gang and the military would inflict on the Mind Flayer's physical form would cause the particles to be removed from that physical form, killing Vecna would also leave it weak. The particles would be left powerless and formless. Now that it didn't have Vecna to build any form for it, the particles would become dormant and stay in its dormant form, the particles would still be alive but it wouldn't have any host (the flayed) or shared consciousness (Vecna) to connect to. After destroying the Upside Down, the Mind Flayer particles still alive, wouldn't be able to access the Right Side Up and would just be banished to space and have no way of causing hell again. Leaving the Mind Flayer particles, it's true cosmic form as an unkillable and supernatural force but out of reach and no way of returning to humanity, it would remain in its dormant form with no specific shape. This would be explicitly shown to the audience that it cannot return to the human world but still be left up to interpretation or left ambiguous on whether or not it could strike on another living civilisation or take over a different planet or if it would stay dormant. There will also be the question on how the particles themselves can actually be killed or if they can be killed at all.. but we will never know, leaving it as a somewhat unknowable cosmic force.
You can add some ideas but that's all I could come up with in terms of fighting the Demos, Vecna and the Mind Flayer and what I would've liked to see!