9. James Rhodes - Captain America: Civil War
Until recently, it was a common criticism of the Marvel Cinematic Universe that the movies themselves had no real stakes - nobody died (Quicksilver aside) nothing mattered, and the status quo almost always clicked right back into place in one or two movies' time. With Civil War and the hypothetical death of James Rhodes, this could have been their chance to answer to those criticisms.
With nothing against Rhodey or Don Cheadle, the death of Tony Stark's best pal would have had an enormous impact on the MCU, giving Civil War a lasting change to the status quo that Infinity War and Endgame couldn't brush under the carpet quite so quickly.
It would be preferable to the "eh, no harm done" shrug we did eventually get when the characters were reunited in Infinity War.