Captain Marvel looks to be preparing Monica Rambeau aka Spectrum to be a new MCU hero in a post-Avengers 4 universe. Marvel has finally unleashed some intel on Captain Marvel after remaining tight-lipped about the Brie Larson solo film. As part of the first official marketing, several new photos have been released, one, in particular, establishes a way for Monica to become a major presence in future films.
Monica Rambeau has gone by many names in the comics (including Captain Marvel briefly), but she is best known as Spectrum. Monica gains her powers through a freak encounter with extradimensional energy. The incident gives her the ability to transform herself into any type of energy and control it. She's also been known to have invisibility, phasing, flight, and much more. Once she becomes a hero, she not only joins the Avengers active roster, but also leads them for an extended amount of time. After leaving the team and passing the Captain Marvel name to Genis-Vell, she goes by Pulsar and joins Nextwave before forming the Mighty Avengers with Blue Marvel. She eventually came to work directly with Carol quite frequently through the Ultimates.
The particular Captain Marvel image of interest for Spectrum fans is of Lashana Lynch sitting in a fighter jet, but it's what's written on the side and her character reveal that sets up Monica. Lynch is playing Maria 'Photon' Rambeau in Captain Marvel, the mother of Monica, the eventual Marvel hero. Maria is one of Carol's best friends back on Earth, but the 1990s setting doesn't point to her being involved heavily in future films. Maria will be a part of Captain Marvel, possibly more than some may expect, as production shifted to New Orleans (the hometown of the Rambeaus) for the final few weeks of filming.
Due to the prequel setting of Captain Marvel, any encounter Carol has with Monica in the film will be when Monica is just a child. But, the friendship between Carol and Maria will likely bond Carol and Monica forever - especially if something should happen to Maria. The key to Monica's set up is the prequel setting though, as it allows Marvel to plant the seeds for her in this first film, but then fully introduce Monica in a future movie. Whether that may be in Avengers 4 or (more likely) Captain Marvel 2, Carol meeting an older Monica gives her the ability to be around for the future.
But who will play this older Monica? Lashana Lynch herself could play both the mother and the daughter, but the adult Monica Rambeau could also be a role Marvel doesn't need to cast until the future, leaving the part wide-open for another star.
The MCU is going to need plenty of new heroes too, and Monica is an obvious choice to include in Phase 4 and beyond. The original cast of characters are expected to be ushered out after Phase 3, and this type of set up all but guarantees Monica Rambeau is in Marvel Studios' plans. Marvel Studios is actively looking to add more female heroes to the MCU, and the Captain Marvel franchise could introduce several. Even though it is Monica getting the set up right now, there's also plenty of interest in Kamala Khan showing up in future installments.
How and when Marvel decides to introduce Spectrum remains to be seen, but there's plenty of options. Since Marvel is skipping Captain Marvel's origin for the solo film, telling Monica's origin in Captain Marvel 2 could be a great way to go. This tactic could allow for flashbacks to Carol understanding her powers, but also give her the chance to mentor another hero and one she has a history with. It also wouldn't be surprising if she goes by Photon (another one of her names in the comics) at first, taking inspiration by her mother's callsign. From there, Monica could go on to lead the Ultimates or Avengers in the MCU. This may mean that Spectrum won't fully be realized for another few years, but Captain Marvel wouldn't be laying the foundation for her arrival if Marvel didn't plan on following through.