I think a way to introduce Ultron without Antman in the movieverse would be to make JARVIS the seed for Ultron. My story pitch would be Starks experimenting on combing the Extermis virus with one of his suits after Iron Man 3 but it result in JARVIS becoming unstable and almost killed him. So Starks gave up on it but SHIELD stole notes from Starks and created a sister AI similar to JARVIS to continue with the experiment. This new AI then goes rogue and become Ultron.
And Hawkeye is easily the most expendable. Hulk is the avengers ultimate weapon, without him avengers have nothing against the big bad foes. Thor/Capt/Iron Man all have their own franchises. Black Widow is there for the sex appeal for the teenage boys. Hawkeye is just there as the non-rich batman gadget type hero. I don't think Renner appeals to the teenage girl demographic but I might be wrong.