Well that depends. Are we talking about a fixed point in the time flow or not? You see a fixed point can’t be changed. Everything else can be and time will heal and correct itself. You mess with a fixed point in time and you have a disaster. Someone lives who shouldn’t or someone dies who shouldn’t or something else happens that shouldn’t or doesn’t happen that should and you have a hell of a disaster in the making.
See I paid attention in Fixed Points 101 unlike certain Time Lords. Now I’m afraid I didn’t do as well in “How not to misplace your companion 101.” They are around here somewhere. Always going running off… As long as they don’t drag any Cybermen or Dalek’s back with them we are fine. The last time they brought back an angel statue and that was a bit of a mess.
So if time stopped for a moment today what would I change? Really? You are supposing I would want to change anything? It also assumes I am can change anything? I mean, honestly, the question is too broad. There are not enough limits for me to give you an answer. Are we talking change in all time or just at that moment so I can fix the-guy-I-am-talking-to’s stupid bow tie [that he thinks is so cool]? Oh wait. I’m not talking to him. He thinks he is the only Time Lord out there.
I can’t give you an answer to that question. Redefine the question and I’ll answer it. Something like “What one thing in all of time would you change?” And by the way that has been tried. Unless it is a fixed point, it just heals itself.
Oh I think my companion is back and… Oh bother she brought Cybermen with her. Got to go.