There’s still a great mystery surrounding Avengers 4 with potential glimpses of time travel and alternate timelines, but this character making an appearance might make sense of these wild rumors and speculation. According to Jeremy Conrad of MCU Cosmic, Avengers 4 plans on introducing the Eternals by way of Kronos, the Titan God of Time. This makes sense for numerous reasons – the biggest being that Marvel Studios is currently working on Eternals as a new franchise and giving general audiences a peek at them, even a little, in Avengers 4 will help give them a great boost. Thanos himself is an Eternal, albeit a mutant among his kind, and Kronos is one of the founders of Titan itself, which appears prominently as Thanos’ dead homeworld in Avengers: Infinity War.
But, most relevant of all, if what the set photos imply is true (time travel) then it would make perfect sense for Kronos to involve himself. In the comic books, Kronos is an embodiment of time itself and is essentially the God of it. The Avengers messing with the timeline in order to stop Thanos might get the attention of Kronos or he might even end up helping them defeat Thanos, similar to the comic book event Infinity Gauntlet in which multiple cosmic entities, including Kronos, confronted Thanos in an attempt to stop him.
Source: MCU Cosmic