Vecna is the end boss of Stranger Things. We already met him in Season 4, and learned his backstory. Unlike the first three seasons, where we were introduced to new major villains which were either defeated or beaten back, Vecna finished Season 4 alive in some form, and the Upside Down began invading Hawkins. Season 5 will be the final showdown between Vecna (boss of the Upside Down) and Eleven.
Max died and was brought back to life in the final Season 4 episode. We last saw her comatose. She will eventually wake up, take awhile to be herself again, but will come around to return to normal and love Lucas all over again. She might have a part in the eventual showdown against Vecna.
Nancy and Steve will get back together. Her current romance with Jonathan Byers was meant just to split them apart for awhile, but they will ultimately end up a couple. They were already preparing for this in Season 4, showing Nancy and Jonathan growing apart due to physical distance, and Nancy/Steve having lots of sexual tension. The fan base wants a Nancy/Steve recoupling, and that's what they will get.
Joyce Byers and Jim Hopper will get married. Eleven will be their legal daughter.
The Upside Down will not be destroyed. Once Vecna is defeated, it will be a much more peaceful place, and no longer a threat to the regular dimension. Access to the Upside Down will be permanently sealed off.
After the defeat of Vecna, Will Byers will feel at peace for the first time since taken into the Upside Down in Season 1.
There will not be any new major characters introduced in Season 5. This season will be meant to wrap up the existing arcs of the characters we already know.
It has not been decided yet if Eleven will finish the season with or without her powers, but she will come out of the season alive.
It is likely that none of the major characters from prior seasons will die in this final season, aside from Vecna. It is much more likely that some minor or few-episode characters will be introduced so they can die, but it is not planned to bring on a major character and then kill it off, like last season's Eddie.
On the gay front...
Robin will finally get with that girl she was flirting with from the band, Vickie (played by Amybeth McNulty). This won't be a major plot point, but it will give Robin that happy lesbian romance ending, and it will draw lots of kudos from the woke crowd who criticized earlier Stranger Things seasons for being homophobic.
Will Byers, the character originally conceived to be the gay teen on Stranger Things, will finally fully confess to Mike that he's gay and that he is in love with him. Mike will be uncomfortable yet mostly understanding, and Will will eventually come around to accept that Mike is straight and in love with Eleven, and embrace his own sexuality without shame. This was the plan all along, but Stranger Things took awhile to get there, so certain oversensitive people got impatient, thus the abrupt turn of Robin being gay as well. To me it was pretty obvious from early on that Will was gay, so this shouldn't have been necessary.