I know HW just came out, but in the live letter today Yoshi-P said that "the second expansion will be released before we know it". So they might be working on it already, and i'm guessing that it will be released in 1 or 2 years possibly (the Expansions for XI were released in 1 year normally, except for the last one which took like 7 years).

Heavensward released one of each role, lets assume the next one will have at least one of each so lets get some ideas.

Samurai - Tank Job - The counter attack / reactive tank. Aside from the regular 1,2,3 combo they will also have a lot of attacks that they can only do after a parry or dodge. The current tanks have a few abilities like this, but Samurai will have alot more and they will have abilities that increase their elusiveness.

Red Mage - Healer - Weaker healer than WHM but lets give them a Refresh for Infinite Mana and possibly a TP version that they can cast on others so they have more utility.

Dancer - Healer - A healer that uses TP to heal

Blue Mage - Caster DPS - This will be harder to pull off because of the ability system in XIV, but it would be nice to have them possibly as a caster DPS.

Puppetmaster - Melee DPS - A melee DPS with a Pet, was fun in XI and would be a great addition to XIV. Lets make it STR base so that there will be 3 int, 3 str and 3 agi DPS.