I think it would be easy to bring him back. I'm not sure what you mean by "explanation of what happened to the body after Godon leaves him". When did Gordon leave him? If he's alive he obviously would need hospital care and would be put there in secret, so escaping would be pretty easy.
I think killing him right away would be a huge diservice to the character
Along the lines of what Dalton said, Dent's status after the film is left ambiguous. I'm in the camp that thinks he's alive and they're faking his death to save his credibility. If Rachel and Batman can survive a fall from the top of a penthouse apartment, and Maroni only breaks his ankles after falling from about the same height, I think Dent's okay.
As long as its a popular character, I think people are okay with it being brought back, especially if it's from Batman's rogues gallery.
What I meant by 'when Gordon leaves the body' is basically 'what happens after the last time we see Dent.' I mean, Gordon's not going to tie himself to TF at the hip, whether he's alive or dead. If he's assumed to be dead but they bring him back, Gordon has to leave him when he's carted off to the morgue..and then Dent has to somehow escape alive...and some explanation of that has to come about and be somewhat credible. If he's really alive and the memorial is faked (or "real" because it's for Dent and now he's TF) and he's receiving secret medical care, that explanation needs to created and pretty believable. I mean, I know that there's an accepted bit of supernatural/'suspension of disbelief' (i.e., Ra's al Ghul...though they avoided mentioning that he's hundreds of years old...), but if Dent is really still alive, then it needs to be explained in a way that doesn't put the credibility of Nolan's world in jeopardy. To just have him appear on the loose and have someone say, "Yeah, we pretended Dent was dead but we gave him secret medical care, put him in Arkham, and now he's escaped"....well, that just doesn't really cut it. All of this argument is because I believe that we're supposed to assume that he's dead. What with him lying there out cold and the memorial service for Dent, I think that to say that we're supposed to assume that he's still alive requires either a wish not to see him dead or a little conspiracy theorizing. Of course, the memorial could be for Dent because now he's Two-Face, but I still don't think that that's evidence that we're supposed to assume he's still alive and that that we're supposed to recognize that that was the reason for the memorial. Thus, bringing him back needs to have some explanation that doesn't seem hokey---at least for the people who'd never thought that the memorial was to honor the "death" of Harvey Dent.
Annnnnyway.... I really don't care if he's dead because I really don't see much of a point of bringing him back. The crux of the Two-Face character is in his downfall. After that, he's pretty boring. Sure, he provides the occasional storyline of "dammit, Harvey, I know you're in there...do the right thing," to which he sometimes proves good and sometimes proves evil. That's interesting, but not really enough for a whole movie in and of itself--the same way Green Goblin 2 wasn't really enough to carry a whole film itself, and thus had to be a fraction of Spider-Man 3. If it's not going to carry a whole film and it's used as a minor plotline, then that makes it far too easy to end up with a fractured story--much like Spider-Man 3 (which did a terrible disservice to Venom in a similar way).
They could bring him back and make him somewhat interesting if they did a story similar to "Dark Victory" (
Batman: Dark Victory - Wikipedia, the free encyclopedia). This would have TF back and waging war on the mobs as sort of a villain vigilante (plus-minus the plot of framing Dent as a cop-killer). This would have him seem to be bad through most of the plot, with glimmers of his good side shining through. It's basically the only type of storyline that has TF be interesting, and, I'll cede, could warrant a whole film. Most of his other storylines pin him as a two-bit criminal that occasionally makes you feel sorry for him when his good Dent side shines through.
So, I guess I'm saying this: Do a storyline like "Dark Victory" as the next film if you want Dent alive; otherwise, let him be dead.