It may sound ideal to try to unite the entire world upon one date, but given the host of variables involved in the process, it is unlikely to happen anytime soon. Since some aspects of determining the new month were left vague by the Prophet, people will naturally differ in trying to interpret what Allah and His Messenger really intended.
Therefore, I believe that trying to achieve local unity is more important, and should be the priority of our efforts. If two neighboring countries were to observe Ramadhaan on different days, it would be less impactful than two neighboring cities determining the month differently.
Likewise, two neighboring cities disagreeing would have less consequences than people in the same city, or the same mosque, or even in the same household beginning and ending Ramadhaan on different days.
The issue boils down to respect for authority. In the absence of any central authority within a locality, whether it be a city, state, or country, any Muslim is free to adopt whatever criteria they want. Muslims, in any locality, must have leaders who are willing to come together and agree on one criteria to adopt, even if they do not agree it is the ideal or most correct method. This is the first step.
The next step would be to ensure that the Muslims respect the decision of those leaders and agree to follow it, even if they are not convinced it is the most accurate method.
Until this occurs, Muslims may find peace in knowing that no calendar in the world is absolute, that the Prophet took into consideration foggy evenings when the crescent is not visible, and that Allah will judge us according to our intentions and efforts.
The unity of this ummah, fasting, breaking, celebrating same day is a desired affair that Muslims should hasten to; and the worst/not permissible part of it is difference of stances of people in the same country, some in fast today, others celebrating the same day, while they are both in the same country; and that which is agreed upon is that the ruling and resolution of the Islamic judge or Ameer or Mufty raises the differences amongst them.
For detailed explanation about the Islamic rulings on Pre-determine of the month by Calender; the varying horizons, global & local sighting; usage of technology; potential visibility & actual sighting; accuracy of visibility calculations; et al, see my book “Fiqh Ramadhan in accordance with the four Madh-hab”.