4. Multiple Ultron Sentinel Drones
As powerful as Ultron is, it would be very difficult to take him seriously as a lone physical threat against the Avengers - especially now that they've got some new muscle in the speedster Quicksilver and the magical Scarlet Witch. Iron Man alone has seen off a couple of armoured metallic foes already, while Hulk has seen off Abomination, Thor has seen off Kurse and an Aether-wielding Malekith, and the Avengers collectively - even prior to recruiting their newest members - have seen off an entire alien army that included gigantic bio-mechanical whale-like creatures. For that reason, Ultron is going to need backup to make his threat plausible and that's where the Ultron Sentinel drones come in. In the comic book arc, the Earth is patrolled by legions of these drones. They determine where people go and what time they have to return and each one of them is a physical threat to even the toughest of heroes. If the Marvel cinematic universe's Earth is teaming with them, the Avengers will genuinely have cause for concern and it would make for a much better and more action-packed spectacle.