3. The Doctor Is Granted Lives From A Higher Power
What if the Doctor's rewarded for saving Gallifrey by the Time Lords? Let's just gloss over the fact that they'd turned evil by the end of the Time War (the Doctor did, after all). It might not even have to be the Time Lords; Bad Wolf would be powerful enough to grant him new life. So would the TARDIS, really. Even an impossible girl could help him, or at least alert UNIT, who might have the technology. That might be a nice twist actually UNIT giving something back to the hero who helped them out so many times over the years. Again, it's a bit of a cop-out: all those years of mythology, the legendary thirteen lives, referenced in some much culture, only to be undone by Timothy Dalton in a silly hat, or Rose Tyler with scary contact lenses.