Arrow star Stephen Amell really wants to see Green Arrow and Superman team-up. Specifically, he wants a crossover pairing him with actor Tyler Hoechlin, who played The Man of Steel in the second season of Supergirl.
Crossovers are an increasingly common occurrence between the various shows making up the collective universe of superhero shows on The CW known as The Arrowverse. It all began with The Flash spinning out of the second season of Arrow, after CSI officer Barry Allen and scientists Cisco Ramon and Caitlin Snow were introduced as side characters in that year's storyline. DC's Legends of Tomorrow brought together a number of popular supporting characters from both Arrow and The Flash, forming a superhero team who protected the sanctity of the timeline. Supergirl, which originally started on CBS, crossed over with The Flash in its first season, before moving to The CW in its second season, prompting a series of four-part events between the various shows.
Amell spoke of his desire for a small-screen team-up between Oliver Queen and Clark Kent while making an appearance at Motor City Comic Con (via Comic Book). "I thought that Tyler did a great job, and I had an opportunity to fangirl over [Smallville star] Tom Welling," enthused Amell. "I would love for Oliver and Clark to eventually meet. I think that that would be incredible."
Unfortunately, a team-up between The Man of Steel and The Emerald Archer is all but impossible at this time, at least from a logistical standpoint. The first chief problem is that - as far as we know - there is no version of Superman on Earth One, where Arrow is set. Tyler Hoechlin's Superman is a native of Earth 38 - the world where Supergirl is set. While travel between Earths isn't impossible, it usually requires a major cosmic upheaval or the assistance of a "breacher" (a metahuman, like Cisco "Vibe" Ramon, who can open portals between worlds) or a speedster with a connection to The Speed Force. As such, it's unlikely that Superman and Green Arrow (at least, the versions we know) could meet without at least one other hero being along for the ride.
Another issue is that Warner Brothers isn't currently allowing Superman to be used in any of their television series, in order to prevent brand confusion with the Henry Cavill version of Superman from the DC Extended Universe. This prevented Hoechlin from making any appearances as Superman during Supergirl's third season, which premiered right around the theatrical release date of Justice League. The same policy also banished Manu Bennett's version of Slade "Deathstroke" Wilson from any further appearances on Arrow, though the character was given a proper send-off of sorts during Arrow's sixth season. All of this, coupled with Green Arrow's uncertain fate going into the seventh season of Arrow, makes any team-up between Clark Kent and Oliver Queen highly unlikely at this time.
Source: Comic Book