0) What if no-one else is real?
It cannot be established scientifically that our universe is real, by definition, so this has to remain an open question. You are entitled to believe that you are the only real person in a simulation created for your benefit (or as punishment). You may even be right.
1) What if you'd been born as someone else?
It's the lottery of life that some be born into riches and others into poverty. The Veil of Ignorance idea suggests that we should build our social systems upon empathy with the legions of unborn children who have yet to draw a straw, and try to at least create conditions of equal opportunity for them. I think this question more than any other separates us morally - some people just don't accept the premise and answer 'well I wasn't' and suffer inequality to exist.
But if you accept the thought experiment then consider also this: could you have been born as an animal? Or as a plant? If you'd been born as a plant, would it matter to you if you were killed?
2) What if everyone acted in the same way as you?
This is of course the golden rule, expressed as a question. And it's a double-edged sword: not only should you not be selfish but also you needn't be too generous. If everyone gave all of their money away we'd end up in a very horrid world where everyone would have to be a monk and no-one got to do what they wanted. 
3) What if you'd never been born?
The rather obvious answer is that the world would have carried on in its usual stumbling historical fashion. But since, undeniably, you have been born, you might as well do something, even if it means having a lazy day in bed.
These are important questions and it's tempting to suppose that a common set of answers might be the basis for a universal morality. But all of this, even the questions themselves, are rationalising-after-the-event (not to mention pompous): As studies into what's called social intuitionism show, when presented with a moral situation we first have an emotional response, and only afterwards do we (sometimes) think through a logical framework for those feelings. In other words, our morals don't come from a set of moral theories, as one of H. G. Wells' characters points out in Ann Veronica (1909):
"There's no sense in morality, I suppose, unless you are fundamentally immoral." textIf there was a science of morality it would have to study this emotional response first and foremost, and evaluate its variation across cultures and across time. For example: does our emotional response to a moral situation change over our lifetime, in response to thinking about questions such as the above, or is it fixed from childhood? Does a baby get its emotional response to a moral situation  from its parents or from its genes?
Further reading: Kohlberg's_stages_of_moral_development, moral relativism, moral universalism, evolution of morality, evolutionary ethics, science of morality, is-ought problem, value pluralism, truth.