In their MCU debut, the Fantastic Four become a Fantastic Five with the arrival of Reed and Sue's son, Franklin Richards. Worries that he might not be "normal" are confirmed in Fantastic Four: First Steps when Galactus recognises that the boy can wield an unlimited energy known as the Power Cosmic (presumably because of the "cosmically compromised DNA" shared by both of his parents).

In Marvel Comics, Doom’s fixation on Franklin is legendary. He views the boy’s near-limitless powers as the key to creating and ruling Battleworld, a twisted reality of his own making as seen in Secret Wars.

As is usually the case with MCU movies these days, The Fantastic Four: First Steps includes both a mid-credits and a post-credits scene. And also sticking to the traditional formula, one is a goofy little stinger, while another has serious ramifications for the MCU.