Maybe not a delay, but rather a staggered set of early releases. Pick out a random selection of players totaling a specific percentage of the player population and giving them a date for early release access. That way, it'd be less of a load on the servers while reducing the severity of problems caused by player overflow and maybe improving the ability to fix those problems when they pop up rather than have every player logging in and having the login servers repeatedly exploding.

Aside from putting more effort into improving login servers, I'm out of ideas on this one. FFXIV is certainly a game suffering from success right now.