Very good question. The only problem is that people will still fight in that ‘one country’ with different groups wanting to impose their own language, religion, ideology, way of life upon the rest or else to separate! So for the dream of One Country or a World without boundaries mankind must first evolve beyond all these things that divide us. At the same time ensure that diversity of cultures, language etc is not threatened. This is only possible through a spiritual evolution by which we discover that the Divine dwells in every creature and we can approach Him or the Truth in so many ways. Only a very wide and all embracing idea can unite human beings without disturbing the diversity. That is possible only in the thought and way of life of Sanatana Dharma. Therefore India has the key to solving the problem of human unity. If it can apply it rightly here, then one day all the countries will unite harmoniously. But right now world is not ready for the Idea of One Country as whichever group takes the lead of governance will try to impose its way upon others and that will be a disaster for others.
Affectionately,
Alok Da