Chicago Fire fans share theories on season ten finale 'surprise' guest star

It's been revealed that another familiar face will be making a comeback for the episode

Chicago Fire fans have a lot to look forward to in the NBC drama's upcoming season ten finale. Not only are the nuptials of Kelly Severide and Stella Kidd set to finally take place, but the episode will see the return of fan favourite Matthew Casey, played by Jesse Spencer.

MORE: Jesse Spencer lands first role following departure from Chicago Fire

On top of all that, it's also been teased that another "surprise" guest star will also be in attendance. But who could it be? Fans of the firefighter drama over on Reddit have been sharing their theories …

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"I hope it's Katie (Severide's half-sister)," one said, adding. "It would be great to see her again, and I can't imagine Severide not inviting her to his wedding."

MORE: Chicago Fire, The Rookie and Grey's Anatomy: What shows have the networks renewed or canceled?

MORE: Chicago Fire boss teases Severide's reaction to Casey's return

Another agreed, writing: "Katie AND his mom NEED to be there at the wedding," which prompted a third to chime in: "I'd love to see them both there. I'd love to see more of Katie and Severide's relationship."

Fans already know that Casey will be at the wedding - but who else?

However, others suggested alternative theories on who the special guest star could be. "Grissom maybe?" one fan wrote.

Someone else was keen on the idea, replying: "Grissom would be a nice touch. Especially with his father gone, and his mention to Stella recently about their strained relationship," although another thought it would be unlikely now that Gary Cole, who portrayed the character, is now staring on NCIS.

The season ten finale will focus on Kelly Severide and Stella Kidd's wedding

Others also suggested that Lauren German's Shay could return in a dream sequence or flashback, or Charlie Barnett's Mills, who departed back in 2015 after just three seasons on the hit NBC series.

MORE: Why these One Chicago stars left the franchise and where they are now

As Chicago Fire co-showrunner Derek Haas told Variety last month, "there'll be at least one surprise guest at the wedding besides Jesse."

"It became a matter of how many people could we get in one day availability-wise, so we put together a wish list," he added, while his fellow showrunner Andrea Newman added: "We had kind of endless wish list that went on pages, but that being said, we also have all these twists and turns in the wedding itself. People that we wanted to get there might not make it."

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