I'd have it where, to save existence and actually wipe out the Daleks for the final time, The Doctor chooses to overwrite his own timeline. Final smile from the Last Doctor, he flips the switch, and everything goes white.
Slowly, it fades into a view of the stars, pans down to Earth, zooms down into a nighttime scrapyard, past a small sign that reads "76 Totters Lane", to finally come to a stop as we see/hear the TARDIS landing. After a moment, the doors open and a CGI William Hartnell and Susan come out...
"Where are we, grandfather?"
"Oh my child, this planet is called 'Earth'."
"Is it dangerous?"
"Hmmm? Dangerous? I don't know, my dear.....I've never been here before. Let's find out, shall we?"
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Wow. That's a terrible idea. It's up there with wiping out the star trek history (to the point of destroying the Vulcan homeworld in the 23rd century) and then having Jim Kirk go straight from cadet to captain in just a few days....