Executive producer for WandaVision, Mary Livanos, has revealed that there was no plans for Monica Rambeau originally.
Here is what Livanos said in SFX Magazine:
To read all the comics, noting every appearance of each character that [she deals] with. This was what led the addition of Monica and other characters to WandaVision‘s cast. Her inclusion in the series was a discovery, and not quite mapped out, but it’s really enriched the show.
Monica Rambeau made her first appearance in the MCU in Captain Marvel as a child. I wonder what role Rambeau will have in WandaVision, and if this is how she will acquire her powers. Monica Rambeau is also known as Spectrum, and the original female Captain Marvel is in my top 10 when it comes to Marvel characters. Monica Rambeau is the biggest reason why I’m excited to watch WandaVision, and I know Teyonah Parris is going to kill the role.
WandaVision will premiere in Disney+ on January 15.