Now with almost five full seasons to its name, it's fair to say that The CW superhero series 'Arrow' has been a huge success. Whilst ratings have fluctuated depending on the season and the story being told, what has been clear from fan response through season 5 is that the show has returned to its original form, as Oliver Queen aka Green Arrow (Stephen Amell) takes on his biggest threat yet in Prometheus.

Stephen Amell teased some big news on his TwitterStephen Amell teased some big news on his Twitter

With the season 5 finale on the horizon, Amell and his co-stars have been giving little teasers as to what to expect, with a slew of promotional material also released by The CW. The latest came from Amell on social media, where he's credited as hugely popular thanks to his constant interactions with fans.

Taking to his Twitter account, the star wrote: "My favorite cameo in the #Arrow Season Finale is not featured in any of the previews. Love this person so much for coming back."

Now of course, fans are speculating as to who the guest appearance could be coming from. Many think that Colton Haynes will be reprising his role as Roy Harper for a short stint, while others think it's more likely Colin Donnell will be bringing Tommy Merlyn back for one of the finale's flashbacks. Then there's the rumours it could be Susanna Thompson back on the scene as Moira Queen.

Stephen Amell continues his superhero journey in 'Arrow'Stephen Amell continues his superhero journey in 'Arrow'

Of course there's always the chance it could be somebody completely out of left field. There have been countless appearances from villains for what fans like to call 'the villain of the week', where they're usually taken out by Team Arrow in the same episode they make their debut.

Whatever the case may be, Amell certainly seems happy with who's making their comeback, so we're sure the fans are in for a wonderful surprise.

More: 'Arrow' Season 5 Finale Will Reveal "Emotional Moments" Missed By Series' Pilot

'Arrow' Season 5 concludes on The CW this coming Wednesday, May 24 at 8pm in the US. Meanwhile the fifth season continues on Sky 1 in the UK.