I'm personally hoping that they implement a branching progression system eventually with a new expansion down the line. Something along the lines where players must work for 5 different but equal sets of gear that work for a unique set of dungeons/raids.
The easiest way to imaging this is using the resistance as an overall gear check.
The trail could go like this:
Hit Lvl 50
v
Entry Level Dungeons
v
Entry Fire| Entry Lightning|Entry Water| Entry Ice| Entry Wind
____v____|_____v_______|_____v____|____v___|_____v_____
Fire Raid | Lightning Raid |Water Raid | Ice Raid | Wind Raid
v
Final Raid
Note: Last boss in each elemental raid will give one different piece of resistance free gear (head/chest/gloves etc.) with a huge stat bonus. These items will be required to progress to the Final Raid.
The difficulty between the elementals will be relatively the same but the time to progress through them will be enough where it's not possible to focus on all of them at the same time. Different companies will priorities different dungeons at first and uniqueness will spawn then fade as the expansion progresses in it's life span.