That might not happen.
Stephen Amell appeared at the Salt Lake Comic-Con and spoke about what would happen if another actor played the role of Green Arrow mentioning he doesn't think there are plans for the character in the movies:
I do not have a right to Oliver Queen. Someone played it played it before me. Someone voiced it before me. Somebody will play it after me. I don’t believe that there are plans for Oliver Queen in the DC Extended Universe, based off things that people have told me, but I would not be upset if that changed at all.
WB already has a Flash in their TV universe (Grant Gustin) and one in their movies (Ezra Miller), so it wouldn't be out of the question to have two Green Arrow characters.
Regarding Green Arrow in the DCEU, a lot of fans have been pointing to Charlie Hunnam playing the character. A while back it was said WB created Charlie Hunnam concept art of the Flash, so it is possible the actor might be on their radar.
Here's a look at Hunnam as Green Arrow through fan art: