Avengers: Infinity War started hot right off the bat as it showed Thanos with the Power Stone already in his hand, later revealed by Thor to be the Mad Titan’s spoil of war in his decimation of Xandar and its protectors. This was an off-screen plot point that sparked a question in a lot of fans’ heads. What if one Nova corps member survived and what if it was Richard Rider? Well, the screenwriters of Infinity War and Avengers: Endgame had this in mind as well, as they considered putting him in the script before it was ever written. According to co-writer Christopher Markus, they wrote a lengthy concept bible detailing all the things they could do for the finale of the MCU:
I remember going through several scenarios of Nova. Thanos could come and kill all the Nova Corps, except one guy. Richard Rider, who then becomes … I think in that scenario he became the herald, kind of like the Hulk was [in Infinity War when he crashed into the Sanctum Sanctorum]. Someone who could come out and tell people. We had everything. Literally every variation we could think of or that was sitting there in the comics untapped. With absolutely no value placed next to them.
Everyone involved made a good decision in making Hulk the messenger to warn Earth of Thanos’ invasion. It keeps things streamlined and concise, Hulk gets a good story beat by being the guy who feels Thanos’ wrath firsthand, and it saved the storytellers the trouble of introducing yet another character we’ve never met. However, I do hope they saved this neat nugget of worldbuilding for Nova down the line. It makes sense for the Nova character we’ve all been waiting for to rise out of Xandar’s ashes.
Among the things they put in their 60-page document conceptualizing Infinity War and Endgame was the infamous X-Men storyline House of M. Writers Markus & McFeely didn’t specify what aspect of House of M they considered utilizing in the film but it’s likely the alternate timeline Scarlet Witch crafts midway in the story. A timeline where history turned out differently from what was in actuality.