Since the expansion involves Ishgard, the dark knight makes sense for launch, samurai won't be around til we get more content involving Doma, which didn't seem to be mentioned in the notes for 3.0.
Remember that Ninja and Rogue had precedent before they were added. So far there's been no precedent for samurai, but there HAS been precedent for Dark Knight (not an actual NPC, but there are thematic elements at work with Ishgard)
Therefore, my prediction is we will NOT be receiving Samurai for 3.0