The Daleks have always been Earth's biggest threat of takeover; Stolen Earth saw the Daleks fly down to Earth and begin a massive war, but were stopped in the end. I received a message a few weeks ago about the working title for episode 17; it was stated that the current working title for the episode was 'War On Earth'. War meaning that two or more things are fighting against each other and Earth being the war ground.
We already know of reports that the Timelords are coming back, but coming back after turning evil. Evil could mean they are more like the Daleks, signifying the chance that they could invade Earth and could be the cause of war. Maybe the evil Timelords want to take over Earth and use it as a new Gallifrey. The Master could be the master mind behind their surprise attack, maybe traveling back in time to bring the Timelords out of the Time War in a similar way to that of Dalek Caans attempt at bringing Davros back in Stolen Earth.
The Daleks will never leave Doctor Who and so a possible way of bringing them back maybe by getting them all out of the Time War. If my idea was true, The Master my have accidentally released the Daleks as well, creating huge problems for series 5 and beyond.
However there always needs to be someone there to save our planet, The Doctor! I think he will some how risk his life for the Human raise even if it means his own raise will die and so regenerates. The problem is The Master wants to get revenge on the Doctor and so tries to hurt the people the Doctor most loves; his companions and friends! He some how gets a car to hit Luke ( Sarah Jane's kind of son.) but the Doctor manages to save him. Donna will die if the Doctor sees her so the Master tries to link them together and he also sends a fire ball to Earth again hurting the Doctor if it harms any humans. Filming has been caught on camera showing Luke nearly getting hit by a car and a scene where a ball of flames hits the world. I wonder if any of my pondering will actually come true- probably not but you can hope.
I can't wait until Christmas; If any one else has any ideas then please leave a comment in the C-Box or below.