@RainGnyu What is currently be done looks more to be trying to sync updates. Basically release the same update at the same time in two different service regions. It is not the same as a Unified Service as there would be the Global Service and the JP Service. This can lead to differences such as, but not limited to, level of communication with the devs and monetization.
The difference is that the Japanese NGS players are basically isolated from the rest of the world. Someone from the United States can play with someone in Hong Kong and vice versa, but neither can play with anyone from Japan despite the two being released at the same time and supposedly getting updates at the same time once NGS is released. This is an issue because the developers are Japanese, they will most likely play the Japanese version and thus get the majority of their feedback from the Japanese version with any feedback they get from the rest of the world having to go through a middle man.
Basically, the Global servers lack a direct communication channel with the developers as everything has to be compiled by a support company acting as a middleman.
Things like the Sword Art Online collaboration is able to release in both JP and Global simply because Sega was able to make a deal with the copyright holder to have the collaboration in both JP and Global. However, this doesn't mean Global will get every collaboration that JP gets. It all depends on whether Sega is able to get a deal working that would allow them to have a collaboration in both servers.
A possible issue in the future is Isekai and Russia. Russia recently banned a number of Isekai series like Konosuba (which we previously had a collaboration with). With Russia being supported by Global, this means that there is a chance that Global can miss some collaborations due to the series being illegal in one of the countries that Global supports. In a unified service, there would just be global, with Japan being part of Global, meaning Sega would not be able to limit the collaboration to a single region.
This is only the tip of the iceberg, but since you mentioned collaborations, I just focused on the potential differences in the collaborations we get if legal issues come into play. Sega did say during one of the prologues that legal issues can prevent some content from coming to Global.