Eccleston not returning as the Ninth Doctor is the most obvious missing element
Especially considering the Moment is trying to show the War Doctor what he'll become if he goes ahead and obliterates Gallifrey. You'd think the first incarnation to deal with the guilt and torment over how the Time War ended would be a pivotal one for the Moment's presentation. Hell, imagine the scene where the War Doctor asks about how many children were on Gallifrey at the end were Eccleston in it. He still hasn't brought himself to finding out, Tennant knows the number, and Smith has forgotten it.
Yeah, I know, it's pointless to speculate on what could have been, especially considering there likely wasn't very much of a chance of Eccleston returning for this. But one can still imagine anyway.