I found it odd that between seasons 1 and 2 of [Arrow](series:720988) that Oliver would go back to the Island that he was stuck on for 5 years. So this got me thinking that the people at DC did that on purpose to set up Stephen Amell's Arrow and Grant Gustin's Flash to connect to the DCCU and lead to a [Justice League](movie:401267) movie somewhere down the road. It all starts with the dates of the season 1 finale of Arrow, [Man of Steel](movie:15593), and the first episode of season 2 of Arrow. Arrow season 1 ended in May of 2013. Man of Steel came out June of 2013. Arrow season 2 started October 2013. During the first episode of season 2 of Arrow we find out that Oliver went back to the island that he was stuck on and tortured on for 5 years.
I asked myself "why would he ever want to go back there?". The answer that I came up with was that DC needed him to be away from civilization during the events of Man of Steel. I next asked myself "why did Diggle, Felicity, and everyone else not tell him about the events of Metropolis?". The answer to that is that Diggle and Felicity did not want him to go out and try to investigate and get involved with Superman. Oliver's friends and family did not mention it to him because they figured that he had already heard about it since they thought that he was only away on vacation.
To bring Barry Allen back into the mix, as we know that as a child Barry witnessed the murder of his mother by what he described as a "blur". Barry comes to Starling City when he hears about a break-in by one man which should have taken a whole team of people to accomplish. Barry is going around investigating cases that have "supernatural" elements to them which is what led him into going to Starling City. Some may then ask "then why did Barry not investigate the events in Metropolis?". My answer to that would be that since we see a young Barry in Arrow then during the events of Metropolis he was either still in school or still in the training stages of his career which prevented him from going to Metropolis.
Jumping back to Oliver being involved in the DCCU, in season 4 of Arrow which will be in fall 2015 (a few months before the [Batman vs. Superman](movie:711870) movie) I believe that Wayne Enterprises will be mentioned in a key way in the show. Oliver realizes that the mayors of Starling have not worked out the way that he would like so he runs for mayor of Starling City. He gets voted into the mayors office with promotional help from Wayne Enterprises. Either the end of the Batman/Superman movie or the beginning of the Justice League Bruce Wayne being the detective that he is will figure out that Oliver is "The Arrow". With Bruce already knowing that he can trust Oliver he asks him if he wants to join the team already comprised of himself(Batman), Superman, and Wonder Woman. To get Barry (The Flash) onto the team, Bruce asks Oliver if he knows anything about the "Speedster" in Central City and then Oliver will have Barry meet Bruce. Bruce will end up liking Barry and feeling that he can trust him will also ask him to join the team.
Please let me know what you think being that this is my first post on here. I would love to see what other ideas people have.