I think there is plenty of room still for interesting class mechanics.

A few people have mentioned it already but Dancer is probably the most obvious example since it could be a melee TP based healer, much as it was in XI. We also have yet to see a lifesteal based healer that uses HP as a resource. There are lots of possibilities there.

Other MMOs have had ranged tanks but positional requirements in this game would make a ranged tank nigh useless unless they had some way to hold bosses in a certain place at range... I think that would be too difficult. A light armoured, magic based tank could be done though; one that uses its MP as a second health bar to mitigate some of the incoming damage (whilst the rest goes to HP as normal). Their skills would focus on restoring their own MP to keep up their mitigation.

For the DPS there are as many options as you can invent mechanics for. Personally I'd be all for more pet jobs, preferably one that is more dependent on its pet than SMN currently is.

The important thing to realise is that at the end of the day the three roles will always ultimately do the same thing. The differentiation comes not in what the job does but how the player engages with the job to fulfill that role. This does mean that class balance becomes an ever more critical component but SE has actually proven to be fairly good at balancing for the most part.