Something that reveals the center of an empire is how strong of a travel connections hub that a place is. In 2017 when I was attempting to get to Romania from Poland by air on short notice I was stunned to find that I could not find any way there for less than $300USD (a year later I would have realized that there are grueling ways to get there via bus for quite cheap). I think it is important to take the following into consideration: you know that you live at the center of an empire if there are many cheap flights in and out of your city to a variety of destinations. If it is expensive or difficult to get out of your city or region, you may find that the area will experience long-term economic decline.
There are many implications if you do not live at the center of an empire. You can become out-of-the-loop in supply chains which limit the variety of products you can access or make them expensive to obtain. You can find it difficult to visit relatives or repatriate any resources and ultimately that place gets starved of new capital or population inflows.
Almost always in Europe, the cheapest place to fly will be London. A lot of this price structure is driven by a high number of people flying to London anyway because they emigrated there for work after their respective countries joined the EU, but there may be something of a strategy to concentrate more wealth there. I wound up in London three times in 2017 in part because it was the easiest and cheapest place to connect to somewhere else. In the US, the cheapest place to fly in general will usually probably be Orlando or Las Vegas because these are the most frequented locations for packaged or family vacations.
The key to any city or region’s long-term success in my opinion is that people are able to get in and out very easily and cheaply. Historically speaking, the countries that epitomize this would be the Netherlands or Uruguay, which are both positioned between other powerful countries and have reputations for strong press freedom and ease of doing business. It will be interesting to see what happens to London after leaving the European Union but I suspect that over time there may be a new hub in continental Europe (Amsterdam is going in that direction but is geographically too small and already overwhelmed with the pressure of huge visitor number increases) but London will see increases in visitors and transactions from greater distances. Canada may be more important in the future if the political instability of the US is a long-term situation (similar to the boost they had in 2001-2002 after 9/11).
Hubs are unfair though because they gradually seize capital from other regions and we must consider how to either create a plurality of hubs or have more transactions take place in a virtual, online environment to save energy and keep people in places where the cost-of-living is lower.