Before the Marvel Cinematic Universe, Marvel films were largely contained in their own universes, which allowed the characters to breathe and be fleshed out at a much slower pace. However, because of this, the odds of the X-Men meeting Spider-Man were virtually impossible. Nevertheless, that didn't stop filmmakers from throwing in some of the best hidden Easter eggs in the entire genre. Thanks to the Multiverse, one Easter egg could potentially change the course of the MCU and all other realities forever.
X2: X-Men United has been widely considered one of the best X-Men films ever made. This was mainly because of how the story, while focused mainly on Wolverine, helped show how Mutants dealt with the stigma of their powers and how hatred could become one of the deadliest weapons. For example, in a particular scene where Mystique snuck into William Stryker's office to steal some files on Magneto's prison, some smaller files could be seen in a brief frame. According to Reddit user Ch0522, this included a file on Franklin Richards.
In the comics, Franklin was the first child of Reed and Sue Richards of The Fantastic Four. While he didn't immediately manifest his abilities, he was always expected to become a Mutant because of how the cosmic rays affected his parents. However, his powers turned out to be more extensive than anyone thought, as Franklin had the ability to build small universes with nothing but his mind. This allowed him to be one of the most powerful Mutants ever, and those same abilities could greatly impact the MCU in ways none would expect.
While the power of Kang is undoubtedly a force to be reckoned with, the option of the Multiverse has offered the opportunity for characters like Franklin to appear at any moment. As a result, he could be the clincher in a battle with Kang or whomever the villain could be in Avengers: Secret Wars. Also, his abilities could be exactly what's needed to fix any alterations to reality or mend the damage caused by Kang and his endless conquering.
Another interesting factor of Franklin's presence is that it would confirm the presence of another Reed Richards. While he was never seen, this would mean there are at least four Reeds, including the 90s version from Roger Corman's Fantastic Four. With these many additions, it would be incredibly easy to also introduce a Council of Reeds for the MCU version to meet. This could even imply further Multiversal situations or a powerful side story where the MCU's Reed recognizes the value of family and abandons the council, like in the comics.
No matter the outcome, the one clear thing about Franklin's presence in X2 is that the option for such a powerful character to appear has grown even tighter. Whether in the MCU or in another reality, Kang could only be the start as more Mutant appearances could mean even greater threats that didn't have the expert upbringing of The Fantastic Four. As a result, even if Franklin didn't appear, the tease has proved that this is only the beginning of powerful characters appearing.