I don't think it would be ruining DK's story if he were actually alive and they were covering it up...
My only real beef with the commonly proposed notion of Harvey being alive and hidden isn't so much that it takes away from Batman's heroics, but that it no longer makes Harvey the traditional "Shakespearean tragic hero" who dies at the end of the play. Brought down by his own tragic flaw of relentlessly pursuing justice (after the scarring, things shifted in the realm of 'personal' justice), Harvey fits the archetype and the plot-based trajectory of Shakespeare's classic characters. I really loved that, and I was quite surprised to see Nolan and co. bring that to Harvey. By keeping him alive, it (partially) invalidates that aspect of the story (yes, the tragic fall is still there, but it removes a well-known staple of this type of character's arc), and I would hate to see that go; it was certainly one of my favorite things about the film.
I find it strange actually, that whenever the notion of Harvey still being alive is brought up, most people tend to focus on the story ramifications of Bats taking the fall, and Harvey's arc itself seems to be a slight afterthought. I've always thought that, with a bit of clever tweaking, the emotional impact of Bats' choice would remain had Harvey lived, but the pure tragedy element of his story would take a blow that I'm not sure I'd like to see happen. I suppose that it's simply a case of loving the story and the themes so much that, regardless of really liking the character, I want his arc to remain as it is, with him dead.
Like you, I don't really expect Nolan and co. to bring him back in the flesh (in the event that it happened, it would take some time for me to come to terms with it, but as always, I trust the men behind the story), considering we've had many clarifications of the fact that he is d-e-a-d. But, Eckhart's change of tone certainly suggests that he's either an awesome troll (which I still suspect), or that he'll be doing something in some capacity. Either way, it's fun to discuss. One idea that I thought was really interesting was the use of fear gas, as that could certainly get quite scary; imagine a demonic, distorted version of Two-Face. Could be neat, if it fits. There's also the idea of some sort of campaign commercial, or perhaps a flashback/dream, which I'm sure, if included, is necessary. Whatever it turns out to be (if it turns out to be... LOL), it'll probably be relatively small, but fun and welcome none-the-less.
All I can say with certainty is that Eckhart's sly smile has caused one massive crap storm, and a lot of discussion to go along with it.
