The resurrection of Quentin Lance following Crisis on Infinite Earths would have brought about major changes to the storyline of Arrow season 7. Beyond allowing him to continue mentoring Black Siren and steering her toward redemption for her criminal past, it would have let him take a direct hand in cleaning up Star City by recruiting its superheroes as recognized agents of the law.
Quentin's death in Arrow's season 6 finale was one of the most heartbreaking moments in the series' history. While he had often been an adversary to Green Arrow, believing that masked men had no business taking the law into their own hands, Quentin came to see that Oliver Queen had truly changed in the years since he had dated his daughter, Laurel, and recognized his good intentions. In the process, Quentin became a foster father to both Oliver and his sister Thea and took it upon himself to try and redeem Black Siren; a super-powered version of Laurel Lance from Earth-2, who had grown up without a father and become a career criminal.
Season 7 of Arrow was defined by Quentin's absence. With the first person to believe in her ability to change dead, Black Siren redoubled her efforts at self-improvement, rallying to become the District Attorney of Star City; an especially impressive feat considering she had no formal education in being a lawyer beyond Quentin's giving her his daughter's old text books from law school. The structure of Star City's government also changed dramatically, as Oliver Queen's chief political rival, Star City Councilor Emily Pollard, became Mayor after Quentin's death and immediately instituted a series of anti-vigilante statutes. This led to crime skyrocketing in the poorest neighborhoods of Star City and the environment which gave rise to a new Green Arrow in Oliver Queen's absence.
Things would have been much better in the new timeline born of Oliver Queen rewriting reality as The Spectre, so that Quentin Lance survived being shot by Ricardo Diaz in Arrow's season 6 finale. Black Siren would have had her efforts at redemption fueled by Quentin's continued and the knowledge that he truly had come to see her as another daughter after he took a bullet to save her life. While she would ultimately wind up in the same position, she'd have a better attitude about her life and likely would have won the acceptance of Team Arrow a bit faster with Quentin's backing.
As for Star City itself, Quentin would have been in a far better position to maintain order after Oliver Queen's impeachment and imprisonment than Emily Pollard. As a life-long cop and Star City resident, Quentin, as Mayor, would have been in a unique position to address the corruption in the SD that made so vigilante superheroes necessary in the first place. It seems likely that he would have introduced the same pilot program that gave recognized superheroes certain police powers which Emily Pollard did, but he would do so in an earnest effort to legitimize superheroes rather than giving them enough rope to hang themselves. In any case, Arrow season 7 would have been an entirely different story with Quentin Lance as Mayor of Star City.