Thanos was defeated in created a rift in the multiverse. The hole in the multiverse will now allow beings from other dimensions to travel throughout the alternate timelines.
The MCU will give the first details of the multiverse in 14,000,605 outcomes through different timelines and they only defeated Thanos in one.
Evidently, the Avengers won in Endgame, it meant Thanos won in all of the other timelines. With the confirmation of a multiverse, it means Doctor Strange was getting a look into other timelines, and evidently, separate dimensions. Therefore, Thanos has full control throughout the majority of the MCU multiverse.
Thanos wasn't lying when he claimed he was "inevitable." There are nearly an infinite amount of Thanos' out in the multiverse. Most of which probably have the goal of world domination. All of the Avengers teams from other timelines are ultimately doomed as seen by Doctor Strange's 14 million failed attempts at defeating the warlord.
With the presence of an immeasurable number of Mad Titans, it would also mean there are countless sets of Infinity Stones among the multiverse. Seeing as Thanos won in all but one of Doctor Strange's futures, it's likely he possesses the Stones in most dimensions. What would stop one of those Thanos' from traveling to the primary MCU timeline and using the Infinity Gauntlet again?
As the Steve Rogers returned them back to their rightful places in the timeline. This could prove to be a deadly decision once interdimensional travel becomes a greater issue. There will surely be a number of villains making their way through that rift in the near future. It might open the door for some new faces to make their MCU debut but it could lead to trouble for the world's mightiest heroes.
