This article from the Stanford Encyclopaedia of Philosophy gives a clear and concise description of the term "citizen" and a good overview of what debate there is around the issue. The most basic definition is: "A citizen is a member of a political community who enjoys the rights and assumes the duties of membership."
The issue in the 17th century was: "the relationship between the concepts ‘citizen’ and ‘subject’". I.e. a citizen with rights, duties and political influence as opposed to a subject to a sovereign autocratic ruler. In the modern age we assume: "that a liberal democratic regime is the appropriate starting-point for our reflections". So we live in a world made up of sovereign states that constitute political communities. In principle every human being on this planet is a citizen in one of these.
However, in a rapidly globalizing world the once iron cast borders of sovereign states are increasingly eroded and subverted by the fact that digital communication, flow of information, funds and goods take place on the social internet. Almost like the printed word after Gutenberg’s printing press, the digital word is now undermining the powers that be and the entire system it is built upon. A new global arena, global village or global community, take your pick, has arisen wherein a person could become part of a global community.
Nothing of the above is exactly epochal. The idea of this diffuse global community becoming a political community however is a fairly recent idea, especially if seen as a political community that may confer citizenship. I do not refer to pressure and lobby groups wanting to change things to make the world a better place, at least not if that is the extent of their ambition. I refer to people becoming conscious of, and actively aspiring to a new kind of political affiliation to a global political community in which they can become citizens in the sense of becoming a: "member of a political community who enjoys the rights and assumes the duties of membership".
My question is, is this possible?
What form would such a political community take?
Which rights, duties and political influence will citizens in this global political community have or aspire to?
Could the idea of a Global City, i.e. a planetary and not geographical location, be the basis of such a construct, moulding itself on for example the Greek City States?
These in "short" are my speculations and considerations. I look forward to someone taking up the debate with me.