Warning: This feature contains SPOILERS for the Arrow series finale.
The resurrection of Tommy Merlyn in the series finale of Arrow resulted in some major changes to the established timeline of the series. Yet there should have been many more changes than those few which were briefly discussed at Oliver Queen's funeral.
Played by Colin Donnell, Tommy Merlyn was Oliver Queen's best friend growing up and his business partner in running the nightclub Verdant when Oliver returned to Starling City after five years shipwrecked. The two came into conflict over the course of Arrow's first season, as a love triangle formed between them and Oliver's ex-girlfriend, Laurel Lance. The conflict became even more heated after Tommy learned that Oliver was the Starling City Vigilante. The two ultimately reconciled in the final moments of Arrow's season 1 finale, as Tommy sacrificed himself to save Laurel during the man-made earthquake caused by his father's Undertaking.
The series finale of Arrow revealed that Oliver Queen used the phenomenal cosmic power of The Spectre to rewrite reality as the multiverse was reformed following the Crisis on Infinite Earths. This resulted in many characters who died over the course of Arrow being resurrected and being revealed to be alive at Oliver Queen's funeral, including Tommy Merlyn. Little was said about how events played out in the new timeline, but Tommy did reveal one interesting fact while talking with Black Siren, the Laurel Lance of Earth-2; Tommy and the Laurel Lance of Earth-1 had gotten married at some point before her death.
Logically, Tommy not dying at the end of Arrow season 1 would have had a domino effect that would have changed multiple events throughout the series, causing them to play out in different ways. Apart from the changes prompted by their getting married, Tommy's survival would have completely changed Laurel's story-arc in Arrow season 2, where she fell into a downward spiral of drug and alcohol abuse due to her guilt over Tommy's death. This, in turn, would have changed the circumstances of how Laurel became Black Canary in season 3, as he would have to have been privy to her secret life.
Tommy living beyond Arrow's season 1 finale would also have had a major effect on the character development of his father, Malcolm Merlyn, whose biggest motivating factor following the failure of The Undertaking was protecting his daughter, Thea Queen, who was revealed to be the product of an affair between Malcolm and Moira Queen in Arrow season 2, after Tommy's death. It seems likely that Malcolm would have tried to reach out to Tommy in secret, at least once, trying to win him over as he eventually did with Thea in Arrow's season 2 finale. While this effort would likely be unsuccessful, it would also fuel an interesting new dynamic between Tommy and Thea, whom he already viewed as the little sister he'd never had while growing up with her and Oliver. Tommy would likely have some strong opinions about his baby sister being trained as an assassin, even if she was fighting alongside his vigilante wife.
The fact that Tommy and Black Siren never met before Oliver Queen's funeral in the Arrow series finale suggests some interesting things about Arrow season 6, where Black Siren impersonated the deceased Laurel Lance and claimed to have been held hostage for the past two years after having her death faked. Tommy must have had to go into hiding for some reason, because there's no obvious way to for why he wouldn't have been forced to publicly reunite with his wife and wind up bonding with Black Siren as she played at being Laurel, the same way she bonded with Quentin Lance. One possibility is that Tommy went into hiding around that same time to sell the idea that he was Green Arrow, which puts another interesting spin on how the Human Target Christopher Chance took on Tommy Merlyn's identity and appeared in court claiming to be Green Arrow to clear Oliver's name on charges of vigilantism.