Spoilers ahead for Episode 20 of Arrow Season 6, called "Shifting Allegiances."
Arrow has been a wild ride throughout Season 6 as Ricardo Diaz rose to prominence in the underworld and Team Arrow broke apart while attempting to defend the city. In "Shifting Allegiances," Oliver was out of commission as the Green Arrow for most of the episode. After attempting to reach out to Anatoly and remind him of when he had honor, Oliver ended up captured by Diaz. The two men agreed to fight each other, with the loser agreeing to leave the city. Oliver seemed to get the upper hand with the option of snapping his enemy's neck, but Diaz pulled a knife and stabbed Oliver in the side. Now he has another scar!
Also, he's in a lot of trouble. The good news is that it was enough to remind Anatoly that Oliver and not Diaz is the man with honor; the bad news is that Diaz decided to take Oliver out of the game by having his bail revoked and getting the corrupt SCPD to throw him in jail. Bloodied and beaten, Oliver was hauled off to jail by the cops in Diaz' pocket, and his trial has been moved up. Oliver will be on trial as the Green Arrow in the very next episode, and things aren't looking too great for him. Diaz basically owns Star City at this point, and far too many people know Oliver's secret identity at this point. The cliffhanger points toward Oliver going to prison unless his friends on the outside can pull off a miracle.
Well, the trailer for the next episode indicates that his pals are going to bring a key character back to get him out of his latest bind. Take a look:
The trailer for the next episode reveals that Oliver's trial will be a media circus, and Laurel/Black Siren will be the star witness whose testimony could land Oliver in the slammer for the rest of his life. If she testifies that Oliver is the Green Arrow, he's not likely to get the benefit of the doubt anymore. The end of the trailer reveals what could save Oliver even if Black Siren does testify that he's the Emerald Archer: Tommy Merlyn dropping down from the ceiling in full Green Arrow gear. Tommy!
Now, on the one hand, it's public knowledge that Tommy died way back in Season 1 and people will undoubtedly have more than a few questions about him turning up five years later, alive, well, and a master archer. On the other hand, he did make his entrance by dramatically bursting through the ceiling to sit in the witness stand, which is the kind of thing that the real Green Arrow would totally do. Oliver has made many mistakes under the hood over the years, but he never fails to make a dramatic entrance.
Besides, Malcolm Merlyn was secretly a master archer with a double life. Perhaps people could believe that Tommy had a double life as an archer as well and he was never really dead. Star City once fell for Roy posing as the Green Arrow; could Star City fall for the same trick twice with Tommy as the Green Arrow? Tommy at least does fit the Green Arrow suit a bit better than Roy did.
All of this said, the Tommy who appears in the next episode is probably not going to be the Tommy who died in Season 1 or even a resurrected Tommy fresh from a dip in a Lazarus Pit. Sure, Colin Donnell will be back, but my money is on the man wearing Tommy's face actually turning out to be Human Target pretending to be a Tommy who never died. Presenting some random civilian as the Green Arrow probably wouldn't work; presenting Oliver's meant-to-be-dead best friend as the Green Arrow gives Oliver enough of a connection to the case that maybe his lawyer could get away with arguing a case of mistaken identity. A key character is coming back, and my money is on that character being Human Target rather than the late lamented Tommy.