Thanos certainly lived up to the hype in Infinity War. Even before most of the stones were in position in his gauntlet, he was easily the greatest threat the Marvel Cinematic Universe has ever seen. According to one theory doing the rounds online, though, he could be supplanted by another big bad in Avengers 4.
Some fans believe that the Mad Titan's adoptive daughter Nebula is about to seize the Infinity Gauntlet and replace her father at the top of the food chain, as outlined here by Comicbook.com. There's precedent for this in the world of comics, as Nebula does indeed wield the all-powerful glove and become an even greater threat to the universe in the Infinity Gauntlet storyline.
There are many reasons why this shouldn't happen in the movies, chiefly because Nebula as the main villain would be an anti-climax after Thanos. No disrespect to Karen Gillan or her character, but even packing the Infinity Gauntlet, she wouldn't be in the same league as the Titan in terms of screen presence, threat and dark charisma.
What's more, having her follow in Thanos's footsteps feels disrespectful to the legacy of her sister, Gamora. Nebula's story arc should involve avenging the death of her adoptive sibling, not replacing her killer as the main threat to the galaxy.