They actually showed concept art for dark knight way back in 1.19 when they first announced the job system. It was originally teased alongside paladin when they first discussed the possibility of classes specializing into two different jobs (both Dark Knight and paladin were provided as the example coming from gladiator). A lot of people in the fanbase have focused on the Final Fantasy XI version of dark knight which had the highest skill cap in both sycthe and great sword and so dismiss the probability that dark knight will be likely come from gladiator and wield sword and shields. FFXIV has focused a great deal on throwbacks to previous Final Fantasy titles (such as FFIII and the none-to-subtle references to FFVI Fernhawles made when discussing the new floating continents). Dark Knight has appeared far more often (and more in more iconic roles) as a sword-and-shield damage dealer.

Beyond Dark Knight, some people caught a screen grab during the music panel which showed a folder labeled "Samurai" on Soken's laptop. In recent conversations and interviews, Yoshida has repeatedly mentioned that a new tank and healer class are also coming (usually in response to concerns that they added rogue and ninja, an additional DPS class before a new tank or healer). The musketeer guild has also been in the game since the original 1.0 release and before E3 they openly discussed a gun-wielding class alongside a dagger-wielding class. We only got Ninja announced for 2.4, so we've been left hanging on musketeer. All we know is that the associated job is "not what we expect," which has led some people to speculate we might get some variant of chemist as a TP-using and gun-wielding healer.

I also think it's likely that Alexander will appear in some capacity, but I think probably as an elder primal like Bahamut and Odin rather than a regular primal. Some have suspected Alexander will have some relation to the Allag Empire believing that the term Allag may be a corruption of "Alex". I'm not holding my breath on that account, but I do think that eventually Alexander will have a fairly large role to play more similar to his role in Final Fantasy IX or XI.