Obviously, you will not be able to call it Justice League as they will not want to hurt the feature film, but maybe it could be called Justice Society or All Star Team-Up or some other name from the comics that would be fitting of the team...
With the Flash already existing in the Arrowverse, the possibilities have now become greater for an event movie to happen on TV. The most logical place for the film is honestly The CW, as they have the larger stake in it, however, CBS would probably get better ratings for it.
Just think though, we have already seen the existence of Green Arrow, Arsenal, Speedy, The Flash, Firestorm, The Atom, Hawkman and Hawkgirl, the Black and White Canaries, The Wildcat, Supergirl, and John Constantine. Still to come, we will see the appearance of Vixen and the creation of Mr. Terrific on Arrow. This would be too strong of a story to be in a crossover or stand alone episode, so it would definitely need an event movie or mini-series of the week.
Now the question becomes, who would the heroes face against? Would you team up the villains they have already introduced in a Legion of Doom or would you find a new, bigger bad? Either way, it could work, but there has to be a universal threat to bring them all together.
Of course, they have already done a bit of this with Legends of Tomorrow, but they are not the top tier characters that a Justice League would feature. However, you still face a major obstacle in not having the big 3 of the Justice League, as there is no way they would risk a TV Justice League outdoing the big screen Justice League with Batman, Superman, and Wonder Woman. This could also work to hurt the television event as well. But again, this is why you do not call it a Justice League.
Finally, they would have to add a new team member that has not been seen before, and that is the other obstacle. You will always want something new for something of this magnitude, but you don't want to spend the whole feature on that new character. Honestly, if he doesn't make it to the screen prior to this proposed movie, Nightwing would be perfect for the role. This way, you get Batman without Batman. Supergirl would be your Superman fixture, and that would leave a need t o fill in for Wonder Woman. A good possibility would be Donna Troy. She could be introduced as Wonder Girl or Troia and would be a great addition. Honestly, it would probably be best to go with Troia though, that way you can acknowledge Wonder Woman without it being blatant.
Anyway, that is my thought on it. What do you guys think? Feel free to leave your comments below...