Season 7 of Arrow may be reviving a key season 2 plot device: Mirakuru. Mirakuru was a serum that gave Deathstroke (Manu Bennett) superhuman strength. In last week's episode, Diaz (Kirk Acevedo) injected himself with a serum that appeared to have a similar effect.
Mirakuru was introduced in Arrow season 2 as a super soldier serum created by the Japanese during World War II. Oliver (Stephen Amell) injected Slade with it to save his life, but it had side-effects that neither character anticipated. Slade became unreasonably paranoid and aggressive, with the Mirakuru eventually making him a dangerous enemy and the main villain of season 2. After Slade's defeat, he was imprisoned on Lian Yu until the Mirakuru finally wore off. When Slade returned in the season 5 finale, he was finally sane again and free of the effects of the Mirakuru.
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In the episode, "Crossing Lines", Diaz and the Longbow Hunters launch an assault on the CDC and lose one of their team members in the process. The Silencer (Miranda Edwards) is captured by Rene (Rick Gonzalez) and Felicity (Emily Bett Rickards). However, this didn't stop the Longbow Hunters from finding what they were looking for. At the end of the episode, Diaz is shown injecting himself with a serum that gives him super strength.
While it's not stated explicitly, it's possible that this new formula is - or has some connection to - Mirakuru. The similarities are obvious, and it'd be a smart way for Arrow to honor a former villain. If the serum's effects on Diaz are a bit different to Slade, it would be easy enough for the show to say that his dose is actually a variation of Mirakuru; it would make sense for someone to want to fine-tune the formula so that it doesn't drive the user insane.
Of course, even if Diaz got his hands on a completely different serum, the consequences could end up being just as disastrous. With his ruthlessness, resourcefulness, and skills as a martial artist, Diaz was already a formidable opponent for Oliver and Team Arrow. Now Diaz has a team of highly-skilled villains to aid him in his quest for revenge against Oliver and super strength, which could have a big impact on his plans; no one in Star City besides Oliver (and possibly the new Green Arrow) is capable of matching him physically.
Whatever is coursing through Diaz's veins, the question becomes what those plans are. With Diaz having claimed what he wanted from the CDC, he may renew his efforts to strike back at Oliver's friends and family immediately, which could put even more pressure on Oliver to escape the supermax. For sure, this development has the potential to make Diaz a far greater threat in Arrow season 7 than he ever was in season 6.