Generally speaking, history is a complicated... thing. It's composed of thousands of factors which interact with one another, which means that changing even one thing in history may have extremely complicated consequences.
For example, a lot of people, if given the chance to go back in time and change something, would kill Adolf Hitler, and thus prevent the outbreak of World War 2. I mean, come on, that's a no-brainer, right? Kill Hitler, millions of lives saved. Well, as it turns out, even such a seemingly obvious choice is... well, let's just say that it's far more complex than it looks at first glance.
So what if World War 2 never had happened? Let's see...
* No Holocaust. Well, most likely, because anti-Semitic sentiments were quite popular in the early 20th century in Europe, so lynchings, persecutions etc. would still take place. Furthermore, no Holocaust = no Jewish migration after WW2 = the state of Israel would not have been created.
* small conflicts would erupt across Europe anyway, most likely due to the fact that nationalism was on the rise and the badly damaged European countries desperately needed something to boost their economy (i.e., a military conflict). The American economy would take much more time to recover to its pre-Great Depression levels (assuming it would manage to recover at all).
* the grand irony: if not for WW 2, it would take much more time for the general public to completely reject the idea of racism and start considering it as something evil. The atrocities of the second World War were an example: they showed how much damage can nationalism and racism cause. Think of it as of a tragic even that shocked people so much that they opened their eyes to see what racism really is.
* Britain would, most likely, still be a colonial empire. Alternatively, the process of decolonisation would start much later and take much more time.
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| The battle of Stalingrad |
* Finally, there is technology. During war time, scientists focus on branches of science that can be used in warfare, paying less attention to less combat-oriented disciplines. Therefore, if WW 2 had never occurred, modern technology would look completely different. The development of, for example, jet engine helicopter and, most importantly, electronics, would take much more time. However, it is quite possible that scientists would invent other things, ones more useful in times not dominated by a total war. So, to sum up: we would have less efficient computers (if we would have any at all), but we would get some... other... unspecified inventions.
And another example: a lot of people may think of preventing the sinking of the RMS Titanic. But then, the sinking of the Titanic lead to positive changes in safety practices. Most notably, the sinking of the Titanic was the direct reason for the creation of the International Convention for the Safety of Life at Sea (which, for example, regulates how many lifeboats ships need to have). More info can be found Here.
So, as you may see, each change in history may have far-reaching consequences. Change one factor, and you change a lot of other things. Furthermore, if we add the butterfly effect into the equation, we may come to a conclusion that it is impossible to predict how history will behave if we change one single thing.
But, from a more personal point of view, one good idea would be to go back in time and prevent the Cretaceous–Paleogene extinction event (the one that killed the dinosaurs), simply because I find the idea of having sentient reptiles to be quite awesome. But on the other hand... It may be a bit difficult to prevent the extinction of thousands of species using only a universal screwdriver.
Also, about the Doctor Who TV show... I heard a lot about it, I even wanted to watch some episodes, but somehow I never got round to it. Anyway, when I hear "Doctor Who", the first thing that comes to my mind is this:
Yes, there is a ponified version of everything.
