For quite a while now, the biggest rumor has been that Hela will be the main antagonist of Thor: Ragnarok. Or should we say the new antagonist? And for quite a while now the rumor is that none other than Cate Blanchett will bring the ruler of the Asgardians underworld to life.
While in the comics Hela is the daughter of Loki (the other antagonist of Thor: Ragnarok), it won't be so in the MCU. Most likely, they are two separate Asgardians, without any connections between them. This latest scoop comes from Geek.com, a website which had quite a string of scoops in the past two days (the most relevant one being the reveal of Guardians of the Galaxy 2's villain).
According to this website, Hela is the one who helped Loki get on the throne from the first place. The two will keep scheming, with Thor being basically oblivious to what has happened - and by the time he realizes it is all too late. In his confrontation with Hela, he loses and his hammer is destroyed. Eventually, he is banished (once again, we might add).
This sounds good, though we would have expected at least a small mention of Surtur for Thor: Ragnarok. But that won't happen. As we can see, it falls on Hulk and Hulk alone to bring the destruction of Asgard (before breaking free from all the magic of Hela and Loki and before allying with Thor).
Basically, from this point of view, Thor: Ragnarok is set to be the live adaptation of Hulk vs. Thor - which is great, by all means. However, when you have an event of this magnitude, why should you choose already known characters over the ones everybody has been dying to see?
Take this rumor with a pinch of salt, even if it makes perfect sense.