Dear Square-Enix. Please preemptively stabilize servers.

[Firepower]

There will be problems, guaranteed. Completely unforeseen random problems, it is impossible to stress test every single mechanic, instance and functionality improvement to the degree that it is going to get swamped with on early release. Nobody could of predicted the issues with the Duty finder on 2.0 release and they ended up separating them into different banks of servers (now there is, Gaia, Primal, Mana, Chaos, Elemental, Aether) In fact I believed in future that they would eventually get re-merged but the game was far more successful than they planned so they ended up having to make more to alleviate stress on the individual lobby servers.

If I had to make any estimations these events are most likely:

Lobby servers to make new characters (Au Ra + new comers) are going to be swamped at prime time periods of the day, if you intend to make a new character do so during the off peak time for your data centre.

Duty finder is also likely to be destroyed, use party finder, also expect a long wait to enter any dungeon, trial or instance, Because the instance servers can only hold so many instances at any time so expect even as tank classes that you will not be restricted by time taken to gather party members but restricted by time it takes for an open spot in the instance for your particular party. I imagine this will also be worsened by smaller parties being able to take up instance spaces in earlier dungeons.

Main story quests will be swamped, I remember on release that early main story quests such as "Kill 5 Ziz" took forever because everyone and their grandma was trying to kill them to progress those quests, I think square will have learned from this and likely provided story quest enemies with faster respawn times, less open world "farm" quests or larger amounts of them available but your best bet is to try and party up for these types of quests to reduce downtime and increase chances to claim/kill.

Some things will be more popular than others, so if you want to avoid the majority of server issues (instances, over populated zones) try and do something less popular. Levelling the new jobs should be ok because there is enough places for a level 30 to spread out to, I would also like to mention that queuing for a dungeon lower than level 30 is possibly a good idea as the majority are likely to be trying to queue Haukke/Brayflox and they are likely to be overpopulated so places like Toto-rak (everyone hates that place anyway) are likely to provide decent dungeon EXP but without a ridiculous wait time to enter, although I would expect Coerthas to be absolutely rammed within a day. Also earlier pre Heavensward instances and zones have already been stress tested and are sufficiently stable for people to play in. But I can't say the same for the new job quests.

Downloading the update even is likely to be a small problem for a lot of people, although their download servers are relatively good, unless you get the download quickly expect to have a lot of traffic slowing down the speed at which you can download.

And finally Definitely expect to see "the world is full" about every time you log in. Personally I would like to see square re-introduce auto log out back again for a small period of time to allow more people to log in.