Mustrum_Ridcully said:
I thought the TARDIS might just have reverted back with the rest of the universe... so if you destroy a Paradox machine, you revert the components to whatever they have been before you begin building it.
Okay, I can buy that, except that the universe reverted to 8:02 on the day the President died, at which point in time the TARDIS had been canibalised into a Paradox machine, and had non-functional time circuitry.
I think I would have been inclined to end the series with the TARDIS non-functional, and throw in a scene where the Doctor tells Jack he can't fix it because he needs 'components X, Y and Z', which aren't available on Earth and can't be fabricated here.
Then, at some point in Torchwood series two, have the team deal with a malavolent band of time travellers (or similar aliens), and have Jack mention he's got to "go see an old friend".
Then, at the start of the Christmas special, the Doctor has just finished integrating the components "Jack brought us", and he wants to take the TARDIS out for a shakedown cruise. Martha, of course, declines to come with him, it being Christmas, so he says he'll be right back. She, of course, doubts him...
And there we go: fixed TARDIS, little nods for the fans of both shows, Martha written out of the Christmas special, and the whole thing fully explained and consistent. It does, however, have the weaknesses of not teasing the Christmas special (not sure if that's really necessary, though), and it also ends the season on a real downer. I suppose if you're going with a big reset button you might as well go the whole way and have the mega-happy ending.