People are worried that a server might fill up before they can get on with their friend because of unbalanced migrations.

So to overcome this my suggestion would be to have a world pass purchased from the adventureres guild. On the world pass you can choose a number 1-20. So lets say my linkshell decided to all choose number 1, then i can tell friends on other servers to also choose number 1. This would guarantee you all a place on the same server 1 (which is unamed for now) until the certain limit of world passes runs out. There can be a option to change number at any time before, in case it does run out.

Lets assume the active player base is 10,000 and the sever limit is 2000 andthe max world passes per number is 1,000.
If at the deadline numbers 1 and 18 have 500 subscribers but numbers 2, 3 and 12 have only 300 subsribers each. All number 1+18 can be in one separate world and 2+3+12 and 100 randoms can be grouped together into a different world, hence making even distribution, so by splitting it into sections initially you can overcome congestion issues with a few servers filling up completely first and the rest remaining dead, while also allowing 50% of free server space. This way there will be no resultant poplated/unpopulated server, which is what SE is trying to achieve from all this.

In the case that a player has not purchased a world pass then they would be evenly distributed across other servers, possibly matching up EU/NA/JP players. If they ended up away from a friend or LS or just recently re-activated then they can request a world invite by making some sort of in game cross-server message to a PC (one time only and only available to those who didnt get a world pass and the LS leader or whoever could accept or refuse).