For the Musical Here are the main characters and who Whedon should pair each of them up with for many of the songs he should include and why Flash and Supergirl
The main stars of the crossover who team up with the rest of the following characters against a band of metas co-led by Zoom/Black Flash and the original Silver Banshee fought by Superman who shall be refered to as Agatha McDougal while Music Meister appears as a secondary villain or neutral character playing the music for the characters to sing the words
Cisco/Vibe and Winn
The two nerds of humor skilled in hacking almost anything by computer and because the both of them in seasons 2 and 3 of Flash and season 1 of Supergirl went through the same struggles with Caitlin and Siobhan and what they did that made their friends turn on them was by forcing and ratting out their personal secrets to others. They are also skilled in making specialized suits for the heroes they work with on their worlds.
Caitlin Snow/Killer Frost and Siobhan Smythe/Silver Banshee
The two women who turned to the dark side betraying their friends for the rebounds and the setbacks they caused them and what they went through that made them become enemies to Barry and Kara as they are blaming them for messing up their lives. While Caitlin is driven into madness by using her powers for so long while struggling to suppress and get rid of them, Siobhan however was able to maintain control of her abilities while her powers within her continue growing by maintaining her sense of Optimism to keep the curse from killing her or corrupting her through the process of sharp ringings in her head. Even though actress Italia Ricci isn't expecting to reprise her role for the musical because of how loud Banshee is with her screams, still, Joss Whedon and CW would say otherwise. Besides having her reprising her role in Supergirl season 2 and Flash season 3's musical crossover would also be the first step to her character's redemption as she will no longer be an enemy to Supergirl as they take on the villainess that was truely responsible for all the hassle she's been going through who is none other than the original Silver Banshee fought by Superman. With Caitlin and Siobhan paired up for a duet in the musical crossover, they'll be sure to get to know each other while they help their friends take down the villains in the two part crossover.
Harrison Wells (Harry from Earth 2) and Jonn Jonzz/Martian Manhunter
Two experts of Team Flash and the DEO who have been through a lot regarding the people in their families even though Wells is lucky that his daughter and that Jonn still lost his family back on Mars. Still one good reason they should still pair up for the musical is because of Jesse and M'gann and how they put up with them during stressful situations and are sometimes not sure how smart Barry and Kara are at thinking before making mistakes and maintaining control of their anger. Jesse Quick and M'gann M'orzz/Miss Martian
As the two girls that Harrison Wells and Jonn Jonzz easily relate to and with Jesse confirmed to appear in the musical crossover next year and with actress Sharon Leal known for her career as a popstar singer, she to shall reprise her role as Miss Martian for this musical two part event and because of how much potential M'gann has compared to Jesse Quick, these two should be a perfect pair while singing with the rest of the cast. and last of all for main characters of this crossover Jay Garrick/Flash of Earth 3 and Superman
Two original heroes before Barry and Supergirl that are seen in the Arrowverse as inspirational mentors to the scarlet speedster and the girl of steel as well as their bold relationship with them in many different perspectives. Now for the villains that CW is to include to make the crossover more epic and comprehensive for heroes and villains of both Supergirl and the Flash
Zoom/Black Flash and Agatha McDougal/Silver Banshee
Two relentless enemies of Flash and Supergirl who have serious impulse control issues brought upon them by the corrupt members of their families and seek to corrupt Barry and ruse Siobhan Smythe into become just like them (as Soulmates). Because many characters of the shows on the CW have such dark backgrounds and traumatic memories with their pasts, With Zoom the villain responsible for making Barry trigger Flashpoint just by murdering his father after Barry irresponsibly made the magnitude decision that costed him both his dad, many cops, reporters, many innocent innocent people on Earth One all because he couldn't handle the sacrifice by ignoring Earth 2 due to the trauma that Eobard inflicted him with on the night he murdered his mother and since Siobhan Smythe is the Silver Banshee from hte NEW 52/Prime Earth comics who is Supergirl's best friend and since the character hasn't lost all of her sanity and is expected to be redeemed on Italia Ricci's next appearance on Supergirl, having her character turn from bad to good by taking on the original Silver Banshee from the New Earth DC comics that was fought by Superman and as an enemy fought by the Man of Steel himself in the Arrowverse before Kara's arrival on Earth and because McDougal's origin in the comics in 1988 was all because of her brother and everyone else in her family's clan rudely spurned her through disruption and murder which just like Zoom from Flash season 2 has overwhelmed her into becoming regardless of any innocent lives destroyed or taken in her crosshairs and highly more aggressive when it comes to those like Superman not minding their own business, having her as Agatha McDougal/Silver Banshee being the villain responsible for the setbacks that Siobhan Smythe went through since she was 17 that made her dislike Kara out of jealousy since they first met in Supergirl season 1 would make her an excellent villain to team up with Zoom against the Flash, Supergirl and all their companions while sharing their lines some of the crossover songs as a duet and for both Barry and Siobhan to stand up to their stalkers who are also "the reverse" to their sanity and everything they stand for and are against.
Next are more villains that should get in on all the action
Atomic Skull (Earth 2) and The Scorcher
Two villains with thermal kinetic powers that Zoom would free from meta prisons on Earth 1 and 4/S and enlist on helping him and the Silver Banshee (McDougal) carry out their plans on getting what they both want so badly and to help keep Flash, Supergirl and their friends including their frenemies from interfering with their plans while they're in motion. Especially when Atomic Skull's powers can affect Kryptonians and Scorcher's powers would heat up the battle with Caitlin as Killer Frost
Kyle Nimbus/Mist and Leslie Willis/Livewire
Two psychotic criminals with metahuman powers to molecullarly disperse into gas and living energy that having both these villains team up with Zoom, Silver Banshee (McDougal), Atomic Skull, and Scorcher would be real interesting since they are usually on both the offensive and defensives of any playing field.
and the villain that is likely to show up to play the music for each song in the next Supergirl/Flash Crossover known as the Music Meister
now one good reason why it would be best to have the Music Meister act as a neutral character while operating the instruments and the music to the songs instead of being a villain that is using music to take over the world is because of how the following heroes throughout both shows have many things in common, their role positions and relationships on both teams, the rebounds and the struggles each of the six characters in the first 3 rows of two have gone through, and how significant it would be to have them stand up to their arch enemies proving that they are not afraid to take them down hard and painfully to make them pay for their hostile and volatile actions and are brave enough to subdue them so that they face justice for all their crimes regarding the lives they've ruined and all the people they murdered out of their serious impulsive issues. Of course they could have Music Meister start off as an anti villain for Supergirl and then have him act as a friend playing the music while the heroes deal with their greatest adversaries that are equal to them at their level and above in the Flash episode of the Musical Crossover.
Other characters that will be involved are
James Jimmy Olsen
Joe West and Maggie Sawyer
Two detectives of Central City Earth 1 and National City of Earth4/S who work closely with Flash Supergirl and Alex
Wally West/Kid Flash and Mon-El
Barry and Kara's sidekicks with potential of helping people even though their friends are highly concerned for their safety when they are both such a handful to them.
Savitar (main villain in season 3 of Flash to appear in one of the shows during the crossover while expected to be mentioned only in the other)
Eve Teschmacher
As Olsen's personal assistant, Eve is likely to be a part of the musical as well and may reveal a possible past friend relationship with Siobhan Smythe as well as why she in the show is often nervous.
Snapper Car
Kara's stress issue heavy boss who is sure to be part of the crossover in the Sipergirl episode of the Musical Event
Sinead, Garrett, Thomas, and the Smythes family
Relatives of Siobhan Smythe whose abduction by Zoom and McDougal motivate Italia's Silver Banshee into escaping through the ventalation shaft of the meta prison she is currently locked up in with her teleportation screams transending her into her aunt's shop where she tells Siobhan every thing about the Silver Banshee fought by Superman during his first 2 to 3 years of protecting Metropolis just before Siobhan confronts the vicious banshee outside the store killing all the officers with her intense sonic screams while merged with Maggie Sawyer revealing herself to be the one that caused all the rebounds and setbacks Siobhan went through 10 years ago back when she was 17 including her father's affair that rused her into hating Kara and after Agatha comes out of Maggie and introducing herself to her self proclaimed soulmate, Siobhan will realize that Kara was never her enemy that it was Agatha who tried to manipulate her theoretically into killing Supergirl just to make Superman regret crossing paths with her since their first encounter and after Siobhan stands up for herself and in defense for the girls from her school that McDougal used to ruse her into antagonizing Supergirl following Barry's entrance into their earth, the evil scottish Silver Banshee will escape with Siobhan's aunt Sinead as leverage and for other sinister purposes in which throughout the musical Supergirl/Flash crossover, Barry and Siobhan will put their differences behind them and team up with Supergirl, Jesse Quick, Caitlin, Cisco and Miss Martian to rescue her family from the Silver Banshee that tried to fully make the Irish Silver Banshee her pawn and soulmate and Zoom, the archenemy of the Flash who Agatha McDougal frees from his imprisonment in the Speed Force to assist her and other metahuman villains into keeping the heroes from interrupting her ritual of regaining her humanity. Other Characters who may turn up for the two part musical crossover
Iris West and Alex Danvers
Sisters of Wally West and Kara Danvers. While Iris' fate by Savitar is yet to be determined, Alex is more likely to show up in the crossover as these two sisters of the heroes are yet to be confirmed to have a duet performance in the musical two part event. and Lena Luthor
While the crossover for next year is yet to start filming in the next couple weeks, we must hurry and get this article of characters over to CW's producers and primary officials so they can include every character in this article for the Musical Crossover to be just as interesting as the Invasion crossover ASAP! So lets get to it!