The sole reason why Limsa Lominsa is extremely congested and popular is due to one simple reason: the Market Board and Summoning Bell is adjacent to the city's main Aetheryte. To wit, this is the only city that has this. Moving forward, if more cities had Market Boards/Summoning Bells adjacent to their main aetheryte, people would spread out more across various cities instead of congregating solely in Limsa, reducing congestion.

As an aside, if new hub cities in future expansions had this, most players would probably idle and hang out here. Compared to Limsa, which is a starting city, a final hub city in 6.0 or 7.0 is gated behind HUNDREDS of hours of gameplay to reach. This means the countless level 1 accounts that shout advertisements for RMT links would not be able to reach these areas. So 1) you create a space players want to be because of its convenience (which is why Limsa is so popular) and 2) you make a city that's difficult for newly-generated RMT accounts to get to. If RMT want to get to these zones to shout their ads, they'd either need to play through the story or pay for story skips (which is a waste of their money because RMT accounts are banned frequently and rapidly).