Marvel have finally released an official plot synopsis!
In Marvel Studios' Thor: Ragnarok, Thor is imprisoned on the other side of the universe without his mighty hammer and finds himself in a race against time to get back to Asgard to stop Ragnarok the destruction of his home world and the end of Asgardian civilization at the hands of
an all-powerful new threat, the ruthless Hela
. But first he must survive a deadly gladiatorial contest that pits him against his former ally and fellow Avenger the Incredible Hulk!
Considering Marvel's previous timescales, we'll most likely be looking at a teaser trailer in April/May to coincide with the release of Guardians of the Galaxy Vol. 2, and full trailer around July to coincide with Comic Con which Ragnarok will most likely have a full presence/cast panel at, and the release of Spider-Man: Homecoming.

Hela is a great character, she's very grandiose and theatrical but at the same time she's a total badass who has the brute strength of Thor himself and vast magical powers to boot. I can't wait to see Cate in this role and in
that
costume.
The previous Thor films certainly haven't been the best that the MCU has put out, but I have a great deal of faith in this one because it's being directed by Taika Waititi who is an insanely talented guy. His last film, 'Hunt for the Wilderpeople' holds a 98% approval rating on Rotten Tomatoes which shows you what he can do given the right material.
All signs are pointing to Ragnarok not only being the best Thor movie, but one of the best movies that Marvel has ever released, which is exciting.

I remember loving the character in the comics but if they do as much of a mess of it as they did with everything else in Thor, it's a shame to waste Cate's talent. I hope they do the character justice.
For every lie I unlearn I learn something new - Ani Difranco

At this stage, we don't really know. All we do know is that Hela, Loki, Skurge, the Grandmaster and Surtur are all confirmed to be in this movie, and in the comics they're all antagonists. But everything points to Hela being the main villain, she was the first character mentioned in the press release (where she was actually described as being the 'mysterious and powerful new villain') and she was the subject of the first (and so far only) concept art that's been released of the film.
So my guess is:

• Hela - Main villain and the one trying to bring about Ragnarok
• Skurge - Hela's crazy devoted henchman
• Loki - Will pretty much be Loki, therefore will be on the side that best suits his own interests (probably conspiring with Hela)
• The Grandmaster - Side villain, will probably have about as much screen time as The Collector does in Guardians of the Galaxy
• Surtur - Final act villain, Hela and Loki will unleash him to finish Ragnarok and burn Asgard to the ground

So is Cate Blanchett's Character going to be the Main Villain in this movie or just 1 of the villains ?

Well cast. She's a good fit, yes.