Warning: This article contains spoilers for What If...? episode one.
Disney litters Captain America: The First Avenger. Carter takes on the title of Captain Carter and leads the war effort against Hydra and Red Skull (Ross Marquand). On the other hand, Rogers becomes the first Iron Man. Though he's left skinny and physically weak after failing to receive the serum, the changes to the MCU timeline allow Howard Stark (Dominic Cooper) to make a Hydra-stomping metal suit. Rogers then uses the suit to help Carter take down Hydra.
Marvel's Loki is full of MCU Easter eggs. The Time Variance Authority in the series needs to hop around to different MCU periods to keep the Sacred Timeline straight, so the show was always ripe for including subtle MCU connections and references. Some have a significant impact on the MCU as a whole. For instance, there were numerous Kang the Conqueror Easter eggs in Loki ahead of the reveal that He Who Remains (Jonathan Majors), a Kang variant, was in charge of the TVA. Then again, others, such as the Thanos Copter seen in Loki episode 5, seem to be included just for fun.
The Loki premiere may have contained an Easter egg that connects to What If...? in a subtle way. The TVA arrests Loki (Tom Hiddleston) for stepping off his path on the Sacred Timeline in the episode. According to the TVA, Loki was never supposed to take the Tesseract and escape in Loki director, Kate Herron, wouldn't debunk theories that the character was Carter, leading to more speculation. Now, What If...? may finally explain the possible cameo.
The Peggy Carter arrested in the Loki premiere may be a Captain Carter variant. In the What If...? premiere, Carter steps off the TVA's Sacred Timeline, choosing not to watch Steve Rogers' super-soldier transformation from the safety of a booth. The small choice changed everything in the MCU, eventually allowing Carter to take the Super Soldier Serum and become Captain Carter. The TVA would never allow a branching timeline like that under its watch. Because of this, the Carter that the TVA arrests in the premiere of Loki could be the same one who chooses to watch Cap without the safety of a booth. The TVA is just trying to stop Peggy Carter from becoming Captain Carter.
Thus far, the theory is just speculation. Even if the character shown in the Loki premiere is Peggy Carter, it doesn't confirm she's a Captain Carter variant. The TVA arrests variants for the smallest possible infractions, so Carter could've been taken in for various reasons. Still, now that the Sacred Timeline has been destroyed in Loki, anything is possible in the MCU. And with What If...? premiering with a Captain Carter episode, it makes sense to include a little tease in Loki. The cameo might be Marvel's tip of the cap to the new Cap.
Marvel's What If...? releases new episodes Wednesdays on Disney+.