From what I'm seeing and hearing, I bet that the expansion will bring 2 new classes and at least 3 Jobs.

The two classes will likely have one use two handed swords (2HS) and the other guns.
Jobs are likely to include Samurai and Dark Knight.

I expect Dark Knight will either be a 2HS dps job or a Gladiator dps job and Samurai is likely to be a 2HS tank job.

Equipment wise, melee dps/tank main weapon sharing has less problems than caster/healer main weapon sharing due to both melee dps and tanks relying on Strength instead of 2 different stats like Scholar and Summoner do.