There was evidence from non-MCU areas of Marvel for Monica using either of her two most commonly used identities (not including Captain Marvel — which she did go by for some time): Spectrum and Photon.
A children's book starring the three main characters of The Marvels from earlier this month referred to Monica as Spectrum. However, a December comic by Eve L. Ewing and Michael Sta. Maria saw a return to Photon as Monica's current primary superhero name.
Finally, Monica's actual superhero name may have been revealed as she looks forward to her MCU comeback.
What Will Monica Be Called in The Marvels?
During an online interview by Black Comic Lords, Monica Rambeau: Photon comic writer Eve L. Ewing indicated that Monica Rambeau will, in fact, be called Photon in Marvel Studios' upcoming movie, The Marvels.
Ewing kept her answers vague, but most importantly revealed that she "did not choose for her to be called Photon in this comic:"
"Keep in mind, comics world, as y’all know, the publishing side and the MCU side do not communicate frequently, but we do sometimes… Let me put it this way, I’ll tell you this, I did not choose for her to be called Photon in this comic, I’ll tell you that. That is all I will say."
While not official, this revelation implies that people from "the MCU side" did require that Ewing use the Photon name for Monica in her current comic series.
Keeping with the MCU's occasional practice of synergy with Marvel comics, this is a strong indicator that MCU Monica will be using the Photon name.
Monica Rambeau Becomes the MCU's Photon
"Photon" as a name is not brand new to the MCU. In fact, it was Monica's mom (Maria Rambeau)'s callsign, just as "Avenger" was for Carol Danvers. Given the grief over her mother's death that Monica began experiencing during WandaVision, taking on a name to honor her would make sense from a narrative perspective.
Much of the MCU's Phase 4 and Phase 5 MCU seem to be dealing with legacy, particularly when it comes to legacy characters.
From Kate Bishop in Hawkeye to Sam Wilson in Captain America and the Winter Soldier, younger and newer faces are stepping into the shoes left vacant by the MCU's original heroes. Fans know that Monica and Maria were close, and as such, it would be a poignant addition to the legacy hero line to see Monica adopt a name of her mother's.
Considering the apparent synergy with Monica's hero name, fans can now reasonably speculate that her costume may match her current Photon costume. Monica's costume is the only one of the main three Marvels that fans have not yet seen in full — only partially on a piece of merchandise. But, what can be seen does seem to match the suit Monica wears in Ewing's book.
Fans can see more of Photon in the Monica Rambeau: Photon comic, the first two issues of which are available in comic book stores now, and in The Marvels when it releases on July 28.