The 22-year-old was out nightclubbing with friends in the early hours of May 27, 2007, when he was seriously injured in a one-punch attack outside the Queens Bridge Hotel at Southbank.
He left the nightclub about 4am before his group was involved in an altercation with another group of at least three men about 100 metres from the venue.
Mr McCormack was punched after stepping in to break up an argument between a friend and another man.
Police believe the 22-year-old hit his head on the ground, and he was given first aid at the scene before being taken home via taxi.
His condition deteriorated overnight and he was taken to hospital, but despite undergoing a number of operations Mr McCormack died on June 3 - a week after the attack - as a result of his injuries.
Detective Inspector Tony Combridge said Mr McCormack's death was lodged in the memory of many Victorians as it was one of the first one-punch deaths.
"No night out should end in a death like this; it's absolutely senseless," he said.
Det Insp Combridge said that 15 years after the fatal attack the McCormack family remained desperate to see those responsible for their son's death held to account.
"He was much loved and popular and had a wide circle of friends," he said.
"Sadly, his parents Bill and Cheryl never got to see the things so many people take for granted – they never got to see Shannon graduate university, get his first job, get married, start a family.
"All of this was taken from them for no reason at all."
In 2013, police released CCTV footage from outside the hotel showing a man investigators are still trying to identify.
He is described as between 165 and 175cm tall with a thin to medium build and light brown or blonde spiky hair. On the night, he was wearing a light purple T-shirt and light blue jeans.
Police are also looking to identify the people the man was with that night.
"Over the past 15 years, it is highly likely that those involved on the night have spoken to someone else about it, and that information may have also then been passed on to others," Det Insp Combridge said.
"All we need is a name and we still believe this case can be solved."
Police are urging anyone with information to contact Crime Stoppers on 1800 333 000, or via their website.
A $100,000 reward was put up to help solve the case in 2009.
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