The trailer of BBC's "Sherlock" Season 4 is out and fans are eagerly waiting for watch the adventures of their favourite detective Sherlock and his loyal companion, Watson.

Starring Benedict Cumberbatch as the iconic detective Sherlock Holmes and Martin Freeman as his ever faithful friend John Watson, "Sherlock" follows the classic tale of Sir Arthur Conan Doyle, but in a contemporary setting.

Known for long intervals between the seasons, the series has received praise from both the critics and the fans for its brilliant plot and amazing acting by the cast. As the fourth season is all set to premiere in January 2017, here are five things that fans might expect in the upcoming instalment, based on the trailer of Season 4.

"Thor" actor Tom Hiddleston to make his debut?

Several rumours have been surfacing on the internet that Tom Hiddleston, who is known for playing the antagonist Loki in "Thor" and other Marvel Cinematic Universe (MCU) movies, will make his debut in Season 4 as the second brother of Sherlock, along with Mycroft Holmes (Mark Gatiss). However, no such official confirmation has been announced either by the network or the show-runners.

Season 4 to be the final season?

Earlier, as reported by UK Cosmopolitan, series creator Steven Moffat had hinted that there might be a possibility that Season 4 will conclude the long-running series.

"I don't know how long we can keep it going. I'm personally willing, but I'm hardly the main draw. I would be moderately surprised if this was the last time we ever made this show, but it absolutely could be," Moffat had said.

However, it can't be said for sure as nothing as such has been confirmed.

Introduction of a new villain?

Apart from its amazing storyline and memorable characters, "Sherlock" is also known for its villains. Moriarty (Andrew Scott) was one of the most influencing characters in the previous season and fans loved him as much they loved Sherlock and Watson.

This season, a new antagonist will be introduced to the series, played by Toby Jones of the "Dad's Army" fame. It is yet unknown which character the actor will play, but it seems that he will be portraying someone with a darker past.

One of the major characters will be no more?

"Sherlock" is known for unexpected twists and surprises in the plot, which often leave the fans shocked. Continuing with the trend, Season 4 will also see some shocking turn of events, and Watson's wife, Mary (Amanda Abbington), might not be there for long.

In an interview with Entertainment Weekly, Moffat said, "John [Watson] has got a wife, and Sherlock likes her and isn't it adorable, and then it all goes to hell."

The show to be darker and more expanded?

Moffat has confirmed that it will be the longest and most thrilling season among the entire series so far. There will be more action, more drama and more suspense, and whatever the fans are expecting from it, the story is going to surpass their expectations.

"We're going to try to take it somewhere we've never been before — a challenging place to go. We might just be going darker," Moffat said.

Watch the trailer of "Sherlock" Season 4 here: