After having watched their M/V for so many hours, listening to that fantastic song, I began to watch the M/V for real, and come up with some theories myself and here you have them:

The doors

From someone who has been following Super Junior for more than 7+ years, could maybe understand this part easier. The doors represent the decisions and paths the individual members have chosen throughout their 12 years as idols. Some of the doors are opened and some are not. Each of the doors could mean something different for the members.

Yesung's door: Yesung opening a door, shows that he chose a path but still has to keep choosing and make the right choice. Here you see him maybe "struggling" with his choice, and he may wonder if it was the right choice he made to have opened the door before. It could also be that he wants to open that door so dearly, but can't because he knows that the eye of the public are following him everywhere (We'll get back to that later)

They also show how he re-opens the door he had once opened and see all the open doors and all the many paths, choices and hard decisions he's made throughout his life. Opening the door to look at his past, he seems as if he wants to redo it all and close some of the doors he has already opened, then they show how his eyes dilated while looking at his past and all the wrong choices he may have done and regrets.

Leeteuk's door: When Leeteuk opens his door, they show an old building and how things are "falling apart". When he covers his mouth, it could show that he might feel guilty for the choices he has made and regrets opening certain doors. Thus, having the group be put under the hate as much as they've been through, throughout these 12 years. He might think that his choices led to the destruction that he feels, and could reflect some deep emotions within him.

The Camera Flashes

The presence of the camera flashes could allude to the presence and influence of the media. How they've been put under the spotlight when they didn't wish to be and how they have to seem happy when they're not. It could also represent how it has been hard for them to do anything in their lives without the public knowing about it. As seen in the video, Siwon is following a girl in front of him but can't approach her faster than he is because it might wake up some attention and start rumors that he doesn't wish to have upon him.

The burning tapes / Static televisions

The presence of the burning video tapes and the static televisions could represent how Super Junior was created in another era and has thus come a long way. Hence the "Old" technology. How they're fighting to keep up with what's modern or not.

Heechul being "locked" up in the room that shows the past, might be why he wants to "exit". He wants to leave the past behind, but can't do so without "exiting"

The connection between Eunhyuk and Leeteuk (?)

The first thing that comes to mind when they swap between Eunhyuk and Leeteuk is one thing:

They've both been under the pressure of being a leader of the group, and thus been criticized for their choices and mistakes. The flames of Leeteuk could show his anger despite being calm and could also mean how much he has literally been put under the fire for his mistakes and choices. Eunhyuk's rain on the other hand, could show his tears and sadness despite his strong appearance. He almost drowned under the massive hate for his scandal while he had the pressure of being a temporary leader. Thus, they both unified and decided to burn away their past and try to move on.

The walking

The shots on the walking could show how they've always walked in each other's paths for either support for themselves or for the member they're following. Simply showing that together, they are way stronger. As Leeteuk once said "We're much stronger together as a team, than individually".

It could also show how throughout 12 years, they've all been walking and aiming towards the same goal, and it still hasn't changed. They've stuck together not only as band mates, but as brothers. They could be laying the path for their other brothers whom may have lost their ways and make sure they all keep up so they can continue to be together for much longer.

Leeteuk and Heechul knows other member's pain (?)

For this gif I have three different point of views

1. Donghae walking alone: Leeteuk and Heechul seem to react, yet differently, when Donghae is walking all by himself. They could be feeling the loneliness feeling coming form Donghae and don't know how to deal with it. Heechul could be choosing to try to ignore it because he knows Donghae is strong enough to get past what he's feeling at the moment. While Leeteuk has the urge to help him but doesn't know how, because he's an emotional mess himself.

2. Heechul understands Leeteuk's pain: Heechul closes his eyes as realisation that Leeteuk know something is wrong with Donghae and knows that their leader will maybe feel guilty for whatever Donghae may be feeling. Heechul, knowing Leeteuk pretty well right now, knows that the latter might keep bottling up his own emotions so he can keep his dongsaengs happy. Realizing this, he closes his eyes in hope that Leeteuk doesn't choose to hide his own feelings and then erupt some time later.

3. Leeteuk is torn between Donghae and Heechul: Leeteuk seems to react when Donghae is walking alone, but also feels the pain coming from Heechul, being their leader, he knows he must do something but doesn't whom to comfort first when he himself is an emotional wreck.

Shindong 

Though it's sad to say, Shindong hasn't been heard much vocally through 12 years, which might be why he is simply walking by the members and observing them from the side. He knows the pain and agony Leeteuk is going through and has been through, but he might not know what exactly to do so he can see his leader smile cheerfully. 

The "Open" sign could mean that, yes Super Junior is "open" but he has always been outside as a bystander rather than being inside to take part of the harmony himself. 

The running

The running could simply show how they want to run away from their past and start anew. But no matter how much they run, it'll always be right behind them. They seem desperate to leave it all behind, and they've showed progress throughout 12 years of being idols.

It could also show how hard they've been trying to keep up with everything as generations have changed, even through their huge amount of suffering for 12 years, and are struggling in their own ways while carrying the weight of the team.

Though, it seems like they realized it's futile to run away, and decide to be strong together to face whatever storm might be headed their way.

They all feel each others' pain

Despite their own pain, they can feel the pain of their members as well. They all have different ways of dealing with it and how to express themselves. They all show some kind of pain through the M/V that is felt by the one that is watching. There are some deep meanings hidden in this M/V that we'll have more fun to uncover when more have made their own theories. 

The Exit Sign

In the video, they show how everything Heechul touches and passes is turned off. One also sees the "Exit" sign above the door he is walking towards. The "Exit" sign could be the most profound symbolism in the video. I have two theories for this

1. It could reflect how he felt during the "Sorry Sorry" era, where he just wanted to "exit" the door and be forgotten by the public. The televisions turning off is probably how he felt it during that time, that he wanted it all to be gone. No proofs, no memories, no pain. But he's also hesitating to "exit" because he knows that the past won't be forgotten that easily and thus, just sits in front of the door to be able to bolt whenever he feels it necessary.

2. The "Exit" sign shows how the Super Junior members are aware that someday in the future, they might need and have to open the door. They will one day "Exit" their public lives and Super Junior as we know it will come to an end. 

When it all comes to all, my other, more happy, theory is:

"Once More Chance" could simply stand for them asking for One More Chance from the public and the video show that no matter how much they've burned and been criticized, they will continue to stand tall.

What are your theories about the M/V?