Cities merging does not need to mean identities merging, or getting a single city centre anyway.
I live in London suburb. I rarely go to central London. It's close for when I want the amenities there, but it's just one centre amongst dozens in London. Heck, my own borough, which is one of 30 in London, has at least half a dozen town centres with their own distinct identities. I've not even visited all of them.
I doubt that will change even with mega-cities. What is considered "low density" regions between the centres will just change. E.g. if you drive through London, it's not like you'll suddenly see farm land (unless you try to - there is some) in between the town centres - it is continuous urban/suburban - but you notice when you come to a new town centre, by the increased density of shops, offices and non-residential side streets. The distinction is quite noticeable even in the inner core boroughs where the "non-town centre" areas may be residential high rises or large town houses instead of small terraced houses, but still noticeable different from the commercial and cultural centres.
I live in London suburb. I rarely go to central London. It's close for when I want the amenities there, but it's just one centre amongst dozens in London. Heck, my own borough, which is one of 30 in London, has at least half a dozen town centres with their own distinct identities. I've not even visited all of them.
I doubt that will change even with mega-cities. What is considered "low density" regions between the centres will just change. E.g. if you drive through London, it's not like you'll suddenly see farm land (unless you try to - there is some) in between the town centres - it is continuous urban/suburban - but you notice when you come to a new town centre, by the increased density of shops, offices and non-residential side streets. The distinction is quite noticeable even in the inner core boroughs where the "non-town centre" areas may be residential high rises or large town houses instead of small terraced houses, but still noticeable different from the commercial and cultural centres.