WARNING: This contains SPOILERS for Doctor Strange in the Multiverse of Madness.
Doctor Strange in the Multiverse of Madness' biggest cameo, John Krasinski as Mister Fantastic, set up what could be the MCU's next major multiverse threat. While the first Doctor Strange had already mentioned the multiverse concept, it was not until Loki that the MCU really started exploring the idea of different – yet similar – versions of the same character. With What If...?, Spider-Man: No Way Home, and now Doctor Strange 2, the multiverse is turning out to be to Phase 4 what the Infinity Stones' storyline was to Phases 1, 2, and 3.
Just like how Thanos wanted to weaponize the Infinity Stones, the MCU is slowly introducing characters that could be a danger to the multiverse by either trying to interfere with it or control it. The first major one was Kang, who was not in Multiverse of Madness, following the introduction of He Who Remains in Loki. In Doctor Strange in the Multiverse of Madness, powered by the influence of the Darkhold, the now-villain Scarlet Witch became the multiverse's most urgent threat, willing to inhabit other peoples' bodies and cause incursions in the process. Doctor Strange 2 also revealed that Earth-838 Doctor Strange ended up causing an incursion while trying to harness the powers of the Darkhold, which shows just how exposed to threats the multiverse is.
With Scarlet Witch dead, presumably, and every Darkhold in the multiverse destroyed, it would seem that the stability of the multiverse is safe, at least until Kang's plans begin to come to fruition. However, Doctor Strange in the Multiverse of Madness' Reed Richards cameo teased that Franklin Richards, son of Reed Richards and Sue Storm, exists on Earth-838. Considering that mutants also exist on Earth-838, as proven by Doctor Strange 2's Professor X appearance, this Franklin could be a mutant just like in the comics. If Earth-838 Franklin is ridiculously powerful as his comic book counterpart, then the multiverse could be in danger now that an all-powerful child who can create and destroy universes at will is about to learn that his father was killed by a witch from another reality.
Franklin Richards isn't a villain, but, as shown by Wanda's downfall, absolute power can corrupt absolutely, especially at times of grief. Doctor Strange 2's Illuminati storyline led to the film spending a lot of time showing Earth-838 and its differences from the MCU, so it is not at all unlikely that the franchise revisits that universe at some point. Scarlet Witch has caused that world great damage by killing its most important heroes, which could have some consequences. One of those consequences is how the other members of the Fantastic Four family, including Franklin Richards, will deal with Reed being murdered by an entity from another universe. Sue Storm may be a mature enough hero to understand that seeking revenge or closure through the multiverse is dangerous and should be avoided, but that is a level of disclosure that Franklin Richards may not have yet.
Doctor Strange 2's first post-credits scene strongly hints at incursions being the next most important storyline in the MCU, which makes a potential Franklin Richards' involvement even more dangerous. Unlike MCU's Scarlet Witch and Earth-838 Doctor Strange, comic book Franklin does not need the help of the Darkhold to influence other universes. MCU's Earth-616 Fantastic Four has yet to debut, but at least a version of Franklin Richards in the multiverse is now properly set up thanks to Doctor Strange in the Multiverse of Madness.
