Some sort of expansion wide threat. Like FFXIV!Jenova or something. The kind of thing that is the big threat across the whole expansion rather than just the first patch and is pretty much resolved.

A feature I'd like to see prior to a new expansion is a level sync for parties like XI had. Sometimes you're hanging out with someone of a lower level and it's gonna be hard for them to level outside of FATEs and dungeons because they're not getting any EXP.