Emiko Queen's plans for revenge upon Oliver Queen seems to be setting up the post-apocalyptic future of Star City on Arrow. There is an odd resonance to this, as Emiko's actions in the past were indirectly responsible for facilitating Oliver Queen's transformation into the Green Arrow. It would be fitting then that she should also take a hand in establishing the future of Star City.
The plot of Arrow season 7 was split between two timelines, separated by two decades. In the modern day story, Star City was left defenseless in the wake of Oliver Queen's incarceration and the rest of Team Arrow being forced into retirement by a new series of strict anti-vigilante laws. In time, Oliver Queen would earn his freedom and Team Arrow would be recognized as a special deputized unit of the Star City Police Department, though many still mistrusted the city's superheroes. The other storyline was set in the not-too-distant future of Star City 2040, where vigilantes are even more hated by the local government and general public than before and are blamed for some unspecified disaster that all but destroyed Star City in the past.
The two storylines largely existed independent of each other Arrow season 7, up until the episode "Confessions." It was here that Emiko Queen and the Ninth Circle enacted a plan to unleash a bacterial weapon on Star City. With Roy Harper returning to town to assist in the mission, Team Arrow was able to stop the Ninth Circle's plan, but not without two security guards being brutally beaten to death in the process. This led to an intense police investigation, as every member of Team Arrow became a suspect in the dual homicide.
Roy Harper was ultimately exposed the killer, having gone into a berserker rage while fighting the Ninth Circle's agents and moving on to the guards when they stumbled across him. "Confessions" also revealed that Roy had died earlier that year while on a mission to destroy the last of the magical Lazarus Pits and that Roy's girlfriend, Thea Queen, used the pit to resurrect him, leaving Roy with the trademark murderous bloodlust of those the Pit brought back. While the Lotus Elixir can normally be used to treat this bloodlust, it proved ineffective in curing Roy for some reason - likely due to how his body chemistry had been altered by exposure to the superdrug Mirakuru during Arrow season 2. This would give Emiko the weapon she needed to strike back, after her original plan was thwarted.
Arrow season 7, episode 20 ended with Emiko luring Team Arrow into a building that was rigged to explode. Before triggering the bomb, Emiko told Oliver that she had stolen the security footage of Roy beating the guards to death and that when she was done everyone would view Oliver Queen and his family as the villains she saw them as. This would explain two aspects of the Star City 2040 storyline - why an older Roy Harper was a wanted man in Star City and why he had sequestered himself on the island of Lian Yu. If Emiko had revealed the footage of Arsenal beating two guards to death in 2019, that would be enough to force Roy Harper into hiding.
As for how else Emiko might destroy her brother's legacy, the season has already seen her impersonating Green Arrow multiple times, and she was once again dressed as her brother when she triggered the bomb that trapped him and the rest of Team Arrow under tons of rubble. Given what we know of Star City's future, it seems likely that Emiko may either make another attempt at releasing the Ninth Circle's biological weapon and "fail" to stop it from a distance as Green Arrow or she will do something else to ruin the Queen Family's name while in disguise. The penultimate episode of Arrow season 7 should reveal just what Emiko has in mind and how it will determine the future of the Arrowverse as we know it.