Alright, so lets assume a player coming from a less populated world which has been decided to merge into a larger world.
The player could be either at the time being;I don't think a blanket merge would not affect the player who is already traveling with his established community. The player would stick to his existing social circle.
- Have a community established from previous world
- Been playing alone
For the player who is playing alone, he is opened to more social circle opportunities to pick from. Oppose to the limited circles from the previous world, which the player did not wish participate for whatever reasons.
Now, Sidious mentioned problems of "two communities" trying to co-exist. It isn't that simple. There are hundreds of communities in a world. Just because a player is part of a world does not mean they partake in all the communities of the world as a whole. Besides, the hundreds of communities in the world itself has to learn to co-exist with each other. Adding more to it does not fundamentally change the dynamics of it.
Here's my personal example; (long story)
I was from Midgardsormr in FFXI, I was part of WeSH for my social outlet, and participated in several endgame shells, including some more specific shells like ShikigamiWeapon.
In 2010, Midgardsormr was merged to Quetzalcoatl. I kept together with WeSH, and a few of my endgame linkshell. ShikigamiWeapon, which was a shell in Midgard that helped people get their Yinyang Robe, was established in Quetzalcoatl as well to help the community.
Did it suck that we lost our world name? Yeah, a bit. But did that mean we didn't have fun together still? No, actually far from it. WeSH is having a great time on Quetz and picked up new members as well.
The only difference I remember noticing right after the merge was having more competition over endgame content, which is natural due to higher population density. Although as usual, over time some endgame shell dissolves, I join new ones, and still kept in contact with my circle back at Midgardsormr. However, I do not know if they were Quetz shells or Midgard shells or even a mix.
Never, have I personally asked new people I meet in game if they were from Midgard or Quetz, because I could care less. For me, a new face is a new face regardless if where they came from. Never was I ousted from a shell because I was from Midgard and not originally from Quetz.
Now, instead, if there wasn't a "blanket merge", where Midgard was merged to Quetz, what would have happened would be similar to when players being to choose a world on a new game. Since people have different circles of friends, they are forced to decide which ones they want to follow. Real life friends who plays, in-game friends, or in-game shells, etc.
To reference again to another personal experience;
I've played Ragnarok Online for while back while it was in beta and continued onto around 2004, which is when FFXI released. A few of the circle of players I knew in RO also wanted to move on to play FFXI.
Some decided to join the Ragnarok server, because of the name. However, some others decided to join other worlds. This essentially tore the circle apart and splitting into much smaller social groups, where the group isn't even sustainable by itself. Eventually what was a dozen players, dwindled down to three players. We had to find a new community to join into, instead of continuing from the one we've established from RO.
Now, if option 4 is picked, it's basically doing just that, forcing players to decide which circle of friends they want to follow. If option 2 is picked, no one needs to decide which circles to follow, because all their existing circles are there. Additionally, they also have the choice to pick another world if they wish. Such as if they don't want to merge into a particular world for any reason, they could try to convince their circle of friends to pick another world.