A while back Moffat revealed that the War Doctor wasn’t originally in the 50th. His role instead would have been played by the 9th Doctor (Sadly things didn’t work out so we never got a return appearance from Christopher Eccleston).
But we do have a tiny tease about how it would have looked thanks to artist Andrew Wildman, who has released some of the storyboards.
It looks like Rose wasn’t always planned either given the young female companion for 9 in these drawings.
Ah what might have been? Although gaining 9 would mean losing John Hurt who gave an excellent performance as the War Doctor. (And is still doing so for Big Finish!)
Personally I’m still mixed about how well 9 would have worked for the 50th in that role. I much prefer him as a damaged Doctor produced purely as a result of the terrible events of the Time War than someone who spent years fighting it.