What Went WrongThere's way too much crammed into this movie. We have to cover the twins' origin story and the creation of Ultron, then build them both into credible threats, and then defeat them all. Oh, and we have to give time for cameos to every other hero in the Avengers' solo movies?
There’s barely enough time to breathe in this movie, let alone let the main cast play off each other like they did in the first Avengers.
We get shortchanged on three fronts: the ensemble cast doesn’t get to interact enough, the villain doesn’t get to do enough to seem like more than a speed bump, and the twins have to info-dump all their backstory so you might care when one of them dies (I didn’t).
How to Fix ItChange the focus, and change the setting. Instead of trying to cover Ultron's rise and fall, cover just his rise: his origin and initial defeat (but not destruction). And instead of flitting around the globe, keep the movie anchored at the castle they assault in the beginning.
Keeping the Avengers in the castle is easy: we let them find the scepter, but they can’t move it. Say its own power is being used to booby-trap it, so if they try to move it without disarming it it’ll blow up and level everything in a 10-mile radius. Stark and Banner will have to stay to study the scepter and disarm the bomb. The other Avengers will stay to guard them.
Meanwhile, the twins weren’t captured in the initial assault on the castle. They escaped and hid, so now they come out to strike at the Avengers, using Quicksilver’s speed for hit-and-runs that let the Scarlet Witch give the team disturbing dreams while they sleep.
We show their backstory by letting the Avengers discover it: they find a scrapbook in the twins' former cell, filled with pictures of their parents and news clippings of the collateral damage caused by Stark weapons. This will give us some sympathy for the twins, and at the same time use Tony’s guilt over his company’s legacy to push him in developing Ultron.
From here, the beats play out much like the original movie: Ultron kills Jarvis and escapes to attack the team, only to be pushed into hiding. And where does he hide? Why, inside the Hydra machinery buried under the fortress. He uses it to build his new body, an army of android servants, and the giant engines he will activate to push up the ground under the castle (and surrounding town) to create his meteor.
Ultron and the twins never need to meet or collaborate. The twins have their own reasons for going after the Avengers, and don’t need to team up. But the effect of both pursuing their own ends will reinforce the feelings of dread the Avengers and their team start getting from the castle, which feels haunted: a gust of wind from a speedster whipping by, the flicker of a computer screen as Ultron hacks another system, waking up in a cold sweat from a horrible dream that you feel is a vision of a dark future.
All the while, this slow build gives us plenty of time for the characters to talk, to play off each other even as the team fractures under the Scarlet Witch’s influence.
Our climax brings everything out into the open: the twins reveal themselves at the same time that one of the Avengers stumbles across Ultron’s robot army. The Avengers are caught fighting on two fronts, until Ultron reveals the second part of his plan: the engines roar to life, lifting everyone off into the sky, threatening extinction for the human race, with the twins' home town as ground zero.
The twins switch sides, Quicksilver sacrifices himself to help defeat Ultron, and they ultimately succeed in preventing the apocalypse, though they lose the scepter in the final blast.
But Ultron is not destroyed. A final shot shows the Cradle being hauled away on a truck crewed by Ultron’s robots, an ominous “Downloading” flashing on a display.
We leave the actual “Age of Ultron” and the creation of the Vision for a separate movie, so we can give that plot the time it deserves.