1. It's Not As Happy Of An Ending As It Lets On
At the end of the episode, we see The Doctor eager to go search for Gallifrey, now hidden in a pocket universe away from the eyes of the solar system. The ending itself makes it seem like a big unknown as to what happens next with the planet of the Time Lords...except we know exactly what happens with Gallifrey. Based on how this adventure fits into the 10th Doctor's time line, Rassilon somehow comes to power and takes over the High Council of The Time Lords, thus launching an effort to bring Gallifrey back to our universe. The Doctor's period of wandering across the universe between The Waters Of Mars and The End of Time did just happen to include his fling with Elizabeth I... which means, theoretically, this adventure happens right before his fateful meeting with The Ood. Honestly, go back and listen to the events he mentions to the Ood when he lands at the beginning of The End of Time: Part I. He mentions Elizabeth I right out. So in the end, we see a hopeful search for Gallifrey or the 11th Doctor. However, we know that even if The Doctor finds it again it's going to be in pretty bad shape due to the events of The End Of Time. Of course, the silver lining is that if this were to happen, a certain arch-nemesis of The Doctor's would finally have a legitimate reason to reappear...