I have made a detailed assessment of an organisation that could act as a pseudo world Government on this website. The best candidate appears to be, the European Union. Incidentally, even pope Francis, in his 2020 Urbi et Orbi message has endorsed such a selection by saying: “The European Union is presently facing an epochal challenge, on which will depend not only its future but that of the whole world”.
To fulfil its role, the EU would have to be transformed from the current confederation status into a full European Federation. If NATO would take up this role, it would have to expand significantly its scope but it lacks sufficient experience. However, when the European Federation is set up and the European Army becomes part of NATO as a whole, merging the two organizations completely is a very logical and perhaps almost a natural step forward.
As far as the USA is concerned, the main obstacle here are the American voters. They are unlikely to agree to constrain their personal and national freedom, and see the USA as just one of the nations within such a federation. It is enough to mention the futility of imposing any sensible gun control in the USA, to see that this option is rather unlikely.
Finally, it is quite possible that none of these options will bear fruit by 2030. In such case, my default option would be China. Just consider that every sixth person on the planet is a Chinese. But do we want Humanity to pass control to the next species with Chinese values alone? It is a challenging thought that deserves a detailed and impartial analysis, beyond this introduction.