Diversity
We have a god, a suited Human, steroid user, monster, girl, and British long bowman. Time to add in a robot if you ask me. Even though the Vision is not technically a robot, he has that A.I. mentality that I think the crowds would eat up. Watching Avengers EMH recently I have realized how funny he can be, but in a perfect way. That dry humor that comes out simply because of the way he talks. He would add in lots of great things for the team. It would also be something completely new for the comic book universe, marvel would be smart to make the first robot hero.
Henchman
Ultron is amazing. Yeah we know, but he really could use another helper. Adding in his second in command vision would be a great addition to the film. Until he turns sides of course. Its just another small reason to have the vision in the film.
Wanda
His relationship with Wanda is a huge part of Wanda herself. Really if they plan to add her, the vision can’t be far behind.
Ultron
Ultron is unstoppable! The Avengers could use some aid. I think there will be 3 confrontations with Ultron in the film. Initial outbreak, a second in between meeting were the Avengers try to bounce back and get beat up. Then the final confrontation and finishing. I would like the first one to be a very small confrontation were Ultron essentially just runs off to hiding. Then there should be an inbetween confrontation with the Vision being sent by Ultron. The Vision ends up losing, and leaves to go back to Ultron. Then the Avengers mount there attack on Ultron, when they start fighting him they just get beat the crap out of. Something triggers in the Visions mind, weather Wanda, or Captain America fighting when he knows he will die, or whatever it may be. And essentially we see a huge showdown between the two of them. Vision loses, but eventually with the help of the Avengers drives him back. He is badly damaged and needs to reboot for a long time. He does however give them information on how to defeat Ultron. Then we see Ultron’s minions take over the world (If the go this route for the story) and the Avengers must stop him. Leading to the vision being an Avenger, however out of commission repairing himself. This would help us with the “were is vision” in the avengers solo films for phase 3.
In closing just throwing out a few reasons the vision should be there with Ultron. If we in fact don’t get the vision in this movie I don’t think we will ever see him. This is the perfect chance to give him big screen time, I hope marvel capitalizes.