The flashforwards in season 7 of Arrow run the risk of revealing too much information about the characters in the series. So far, the flashforwards have already confirmed that several key characters on the show will survive for another twenty years.
The first five seasons of Arrow explored a five-year period in Oliver's life, beginning with the accident on the Queen's Gambit and ending with his rescue on Lian Yu. Each season accounted for one year of flashbacks. After season 5, Arrow shifted to occasional flashbacks to reveal the backstories of other important characters, like Diaz. An even bigger change came in season 7 with the introduction of flashforwards, which take place in 2038 and are centered around Oliver's son, William. The flashforwards will remain until the end of the series.
After four episodes of season 7, a handful of characters have already been confirmed to still be alive in the future timeline. The premiere confirmed that Roy Harper has been living in exile on Lian Yu. In the latest episode, both Dinah and Rene were revealed to be alive as well. Felicity is supposedly dead, but it's been mentioned that she starts a company called Smoak Technologies, which means she'll survive on the show at least long enough for this to happen.
Still, there is a slim possibility that the future timeline isn't fixed and is subject to change, though this scenario would depend on the inclusion of time travel. Some believe that the new Green Arrow is actually William from the future, which would tie both storylines together. The most recent episode of Arrow includes evidence that supports the future William theory. If Arrow were to go down this route, William would be able to alter the timeline and essentially write everything that happened in the flashforwards out of existence.
The main issue with the flashforwards is that if they are fixed, unchangeable events, they may reveal more than audiences need to know about the characters. Their presence in the future timeline is confirmation that no matter what happens to them in the present day, they won't die. This is particularly problematic for a show like Arrow, considering its reputation for killing off main characters. Through six seasons, Arrow has killed off Tommy, Moira, Laurel, Malcolm, and most recently, Quentin. Arrow has become a show where no one but Oliver is guaranteed to survive every season.
Thanks to the flashforwards, this is no longer the case. If your favorite character is Rene, Dinah, or Felicity, you won't have to worry about anything happening to them in the present day storyline for the rest of the series. The flashforwards can put a limit on what the writers can do with the characters. In the premiere, William told Roy that they hadn't seen each other since Roy left Star City with Thea in season 6. This means that if Roy ever does return to Star City at any point in the series, Arrow will have to deliberately avoid the two sharing a scene just to avoid contradicting the flashforwards.