With the conclusion of Arrow and the Flash, many opportunities have also opened for the DC TV universe. Particularly the new arrivals Supergirl, Legends of Tomorrow and most exciting one, Hawkgirl.
There has been some clarity given on this topic by none other than the Arrow!!! And I Quote...
"Now, Constantine was an NBC show. I’m on The CW. I had had discussions with DC… so not NBC, not CW… but with DC Entertainment about guest starring on Constantine when it was still on the air. And that was and is still on the table. So we have Flash, we have Legends of Tomorrow, a new show which is directly connected to the Arrow universe, we have Supergirl… she got picked up, directed by my good friend Glen Winter, that pilot, I can’t wait to see it, I’m sure it’s going to be amazing… there’s Gotham obviously, and I’m missing something… Titans, there you go. So again I wouldn’t worry too much about the movie universe, because we have no shortage of options for me and other characters to populate."
But still many open endings have been left and there are some parts which are not connected properly. It's still not sure whether Supergirl and Gotham belongs to the Arrow-verse. In the case of Gotham it is illogical whereas in Supergirl there needs a little modification to ensure the connection.
Lets consider someway we can do that.
As per the trailer we can see that Superman is an already existing entity. Although lets assume that his reach is only till Metropolis and he is not at is peak right now. In fact he has just began his Superhero Career. So at this point he wouldn't be much famous in Starling City and Central City. Also it's important that he doesn't face any major threats as of now. Then in the current timeline in Arrow and Flash, the residents in the two also have some disturbances due to the events in Supergirl. So it's much easier to maintain the continuity and if things turn out better help in the future stories of her.
With that lets move on to the next anticipated character which has been hinted for a long time.
2. Green Lantern
Anyone who paid good attention to the Dialogues of Arrow and the Flash. You can see they are taking up Hal Jordan version of Green Lantern. So, if they manage to get the sufficient budget by the Arrow Season 4, It would be possible to make it the best work. For the first half season one we Should see Hal's Journey in the Green Lantern Core and the Gradually the corruption of the core with fear. If Supergirl and Green Lantern run parallel with timely interaction then by season two in Green Lantern other Cores must also be introduced. So by season three they could make a different move by making both the shows collide by shifting the focus on The Red Lantern Core which is very much associated with Supergirl. So Supergirl can focus on her transformation to the Rage machine, and Green Lantern can focus on Atrocious and his personal conflict to fight against an ally.
Now for the least possible but the most important and kickass one.
By this time the Creators must make sure that all the characters in this hypothetical situation acknowledges that there is a myth about a masked man in Gotham. It would also be good if they implement this to the relationship between Oliver and Speedy with Ollie knowing some tales about a guy who's assistant was killed brutally( Batman and Jason Todd) which finally resolves when he meets the real person during a Crisis which threatens most part of their shared world.
Lets also consider a different dimension to the same issue.
4. Batman (Alternate Edition)
In this edition there opens a sub-plot where Malcom searches for more Pits as the one in Nanda Parbhat is depleting. He then learns from Nyssa that there is someone who know the location of a secret one( not Constantine) which is only accessible to Talia, her half sister and also learns that she was last spotted in Starling City. On the other side It's revealed that she is pregnant with a child while she was using the last pit. Also she was in Starling City to recruit Assassins for fighting against Malcolm. Both the groups fight in Starling City and Team Arrow intervenes. They learn that the father of the unborn child was listed by Ra's Al Gul as a heir before Oliver and was his most favoured Disciple. She is in labour while the city faces turmoil and that time a masked Vigilante who resembles the myths about the guy in Gotham. He finishes the forces Malcolm had sent for Starling city and reaches the Arrow cave and Claims to be the father of the child. The aftermath of the events could for up the Season one with flashbacks showing his origins the same way as Arrow with a Main foe mostly Penguin.
Now lets discuss something which is both Possible and Impossible
This one is easy if Supergirl get established nicely. As I mentioned Season one must have him as a Local hero rather than protector of the earth. And Season one will deal about such a moment which makes him face a Global threat.
In the end, putting aside all these theories the basic fact is that its all dependent on time and on how Supergirl turns out.