And seeing as we know so little about the 8th Doctor (not counting audios, which Moffatt could've discounted, had he wished), he could've very easily been written as a Doctor who went dark, especially while the Time War went on. Having said that, I'm perfectly with DOTD as it was, other than wishing for Eccleston to have taken part.
Umm but wasn't Davison younger than Eccleston? maybe even Tennant? And isn't Hurt older than Hartnell was?
^Although Hurt's Doctor had the face of a young man when we saw his newly-regenerated face in Night of the Doctor. Capaldi is as old as Hartnell was in 1963, though looks younger.
The million dollar question is this: Had Eccleston said YES, would Moffat had written it with the War Doctor STILL, and had him meet all THREE of his future regenerations... ...or would Eccleston taken the place OF the War Doctor, meaning no John Hurt but rather him in that place with very minor script changes? It could work either way. He'd fit into the whole :TRhe one that regrets" and "The one that forgets" part as "The one that hates himself" or some such. But he also could have worked in the Hurt role. Guess we'll never know.
Eccleston would probably be "The one that encounters Daleks after they were all supposed killed and wonders 'What the hell was it all for?!'"
I'm inclined to believe that Eccleston would've been the third Doctor the War Doctor would have met. The "one who regrets" line would have suited him the most, really, or it could've met "the ones who regret" and I like to think that the whole time fission effect would've been a creation of Bad Wolf, thus making the Ninth Doctor from somewhere at the end of the episode, oddly enough. Nothing conclusive, but thats what I feel would've happened.
I strongly suspect that much of the plot was written after the major casting was known (so Moffat could adapt to either Eccleston or Tennant), but I tend to agree that Eccleston likely would have replaced Tennant rather than Hurt's Doctor. I think Moffat wanted his own Doctor for the role. If it had been McGann or Eccleston, fans of those characters would probably have been upset at the dramatic character revision necessary to make the character work. It had to be someone with a bit of an unpredictability who was "different" from the other Doctors. Plus, an established Doctor isn't a secret (one he will take to his grave).
Even though I did originally say it seemed as though the War Doctor was originally written for Eccleston, I got to admit with more thought switching the two would have had significant impact to the story. So yeah, I'm sure Moffat intended all along to feature a Doctor fighting the Time War meet his future selves.
Moffat admited early on that Tennant was always gonna be in it, being the most popular NuWho and all that. Getting Eccleston would've been a bonus - and there's the odd rumor that he'd had two scenarios planned out for the special, one WITH Nine in it, and one without. And we saw the latter one, obviously, if thats to be true. While I agree with you, I would still have liked it enormously if it were Eight instead of War Doctor in it. Not only because of prospect of Paul McGann in a meaty, full-center role, but because we know that Moffat would aknowledge the suffering that Doctor went through in the audios and would add them in his characterization accordingly. Eccleston being in the War Doctor's place is also an idea I'd like, and he's my favorite Doctor, but I think it wouldn't make that much sense, unless he'd been recently regenerated before exterminating Time Lords and Daleks alike. In the end, we still got a pretty good John Hurt as a Doctor, which if fine enough, really. But I wouldn't have been phased at all if it'd been either 8 or 9 in that role - in fact, it'd be more preferable, from a writing perspective.