Warning: SPOILERS for Justice League: No Justice #4 & Green Arrow Annual #2
Green Arrow doesn't have any powers, but he does have the means to destroy the entire Justice League. DC's most recent event miniseries Justice League: No Justice ended up changing the status quo in several ways. One of its most unexpected qualities was becoming a terrific Green Arrow story, leaving Oliver Queen as the last hero on Earth. And a good one, too.
The finale to No Justice brings all plot lines to a close (for now) including Oliver's own mission, performing admirably in the absence of the superpowered Justice Leaguers. As a "reward" for acting as Earth's last line of defense, Oliver is given a weapon to take out literal gods. If the group ever goes too far, the Green Arrow now owns a failsafe to neutralize the entire Justice League.
The only problem is that Oliver couldn't be less interested in this massive responsibility.
The massive moment for Oliver occurs at the very tail end of Justice League: No Justice #4. In a series of panels that tying all the various loose ends of the miniseries together, while setting up the interesting future for several different Justice League teams, Martian Manhunter comes to Oliver alone.
J'onn tells Oliver how proud he is of his heroism, and offers the emerald archer not a seat on the League... but a mysterious box.
J'onn informs Oliver that they need someone honorable to preside over this box and keep it safe. According to the Martian, the box is the most dangerous thing on the planet and it can destroy the Justice League if the need should arise.
It's a momentous occasion for Green Arrow, that could change the course of his life. It isn't given much context in No Justice #4, but Green Arrow Annual #2 shows the exchange in greater detail. The emphasis is placed on how the box will relate to the Green Arrow series moving forward, as well as the DC Universe at large. The moment where J'onn gives Oliver the box is expanded to show the whole breadth of their conversation, and just how badly how Oliver doesn't want to own this powerful weapon.
Perhaps the most interesting wrinkle is when J'onn tells Oliver that they are the only two people who even know about the box's existence. Meaning that Oliver not only has the means to kill the entire Justice League, but apparently must also keep a huge secret to own it.
It's the secret of the box that might end up being a bigger deal for Oliver than anything else. The Green Arrow has never been much of a team player. Yet since the New 52, DC Comics has really doubled down on the idea of Oliver as an outsider to the Justice League. Oliver has been denied League membership several times. Now his outcast status does give a nice outsider's perspective to the League, if they go down the dark side.
However, keeping the box a secret also means keeping the Black Canary, a member of the Justice League of America and the love of Oliver's life, in the dark. The lie of omission to Dinah is appears to bother Oliver more than having a kill switch for Superman, Wonder Woman and Batman.
It's probable that keeping the secret from Dinah will be the source of most of the tension in the main Green Arrow series when it comes to the box. It's obviously a matter of time before Black Canary or another ally of Oliver finds out about it, but the secret itself will probably be the immediate fallout. It may even threaten to break up Green Arrow and Canary's wonderful romantic reunion.
The mystery and power of the box still lingers, since there's no way that DC Comics is introducing such a huge and powerful artifact into its universe, and not planning to do anything with it. It might be years until the box is used, but it almost certainly will be in the near future (if it isn't stolen by someone else).
Not to mention that DC needs to confirm or at least explore what exactly the box is, as Oliver and the reader are given almost no explanation for its existence. The smart money is it being Pandora's Box from the New 52. That box was destroyed during the Trinity War event, and hasn't been seen since... but nothing stays gone forever. If it is a repaired Pandora's Box that would explain why no one but J'onn and Oliver know of its existence.
Regardless of the box's origins, its existence just made Oliver Queen so much more than a guy with a bow and arrow. Although Green Arrow fans have known that for quite some time.