Update 20150715: Now as I read up on the current greece debt crisis, I think this is a dumb idea. Without a sovereign currency which can be simply devalued and printed to keep government running the soverignity can be taken away by outside lenders if a common currency is used.
A sovereign currency is essential to preserve local independence. Then the government can simply print its way out of debt. Infact with monetary sovereignity a government doesn't require any taxes or borrowings or other income to keep functioning. It can even simply do away with all taxes and print all its expenses. A monetarily sovereign government cannot live beyond its means (because it is its own means, its means is practically infinite). Government austerity is the method for widening the gap between the rich and the poor. Government expenses on the other hand reduce the gap between the rich and the poor, though the rich will not feel the pinch because they are not directly sheared. Internal sovereign government debt and deficit is a fictious thing that can be wiped out by government fiat or just ignored. A monetarily sovereign government can never be forced into bankruptcy. Social security and other governmental expenses of a monetarily sovereign government can NEVER go bankrupt because the government can ALWAYS print more money to pay it. In fact money is created when the government pays its bills. Government debt is NEVER the same as personal or business debt. Increase in internal government debt decreases the gap between the rich and the poor. The single most valuable asset any nation can own is their monetary sovereignity. Government Debt/GDP ratio is a meaningless ratio. Internal government debt is a meaningless number. External government debt can be settled by default or postponed anytime. The spending by monetarily sovereign nations is constrained only by hyper inflation. It is not constrained by ordinary inflation. If you do not understand monetary sovereignity then you do not understand economics. Taxes do not support (monetarily sovereign) government spending. It gives the illusion that people are paying for government expenses. This is a dangerous illusion as it results in meaningless taxes, convoluted accounting systems to track those taxes. It also makes people believe that government spending needs to be curtailed for prosperity. Government spending increases the prosperity of a nation. It is governmental taxes that it does not need that reduces prosperity. A monetarily sovereign government can NEVER go broke.
Governmental accounting practices should transition to a single entry method of accounting instead of the current double entry. This is because it should only account for expenses because it is its own income. Its expense plan is a plan of how much money it is creating by fiat for that year. Governmental budget should be renamed as Governmental expense plan. This will eliminate the term governmental debt which is a meaningless phrase that doesn't affect anyone, not even the government. There is no need for double entry as a monetarily sovereign government doesn't need any income. So it doesn't need any taxes to pay for itself. So there is no need for any entry on the income side.
It has been feared for many centuries that machines will replace most people at work. However it has not yet come true so far. Just because some fears have not yet come true does not mean that it will never come true.
If you read up on the advancements in robotics and computers you will be stunned. I expect that within a century every thing from brain surgery down to cleaning toilets will be done by robots. (If not within a century then certainly within a 1000 years.)
This is not something to be feared, but something to be welcomed. However we will have to upgrade our economic systems for a future where people outnumber work.
The best alternative I have been able to think/read up so for is Unconditional basic income (UBI). Every month the government credits EVERYONE with an EQUAL amount. This is to keep the economy moving. The capitalists will welcome it because people will have money to spend on their goods and all the credits will end up eventually in their accounts. The govenments will welcome it because it will give them more control over the people. The banks will welcome it because all the amounts will flow through them. People will still work because they want to work not because they need to work. Professions like entertainment, art, culture, research where automation doesn't make any sense will become more important. People can spend more time studying and researching instead of doing menial jobs.
The above is my current understanding after reading and thinking extensively. I reserve the right to change my mind anytime after further reading and thinking. Below is the previous idea that I now think is dumb.
In this essay I propose a world monetary union in which all the countries of the world move towards a common universal world dollar currrency - the univos that is to be printed and maintained by a world body like the world bank.
Reasons for forming a world common currency.