I'm a little torn. I enjoy the 4 man dungeons for easy content access, but I'd really enjoy a new role for tertiary classes. If an expansion adds 5-10 more levels, I would like the new light-party zones to be 5 player. This would open up a lot of strategies for new classes like THF, RDM, NIN, etc. Then again, it's hard enough to rely on 3 other party members. Having to rely on a fourth to know how to play might be hard. Though, if a third order role could easily be replaced with a decent DPS class it could be OK.
Alternatively, if there is no room for tertiary classes or 4-player groups will remain the dungeon standard, maybe it would be best to provide a way for any melee to tank light party content. You could check a box that you want to tank and be forced into your version of Shield Oath. Raid content could force everyone to more traditional roles. I enjoy light armor tanks, but I can understand why it could irk some people to see a nearly naked ninja tanking a giant raid boss.
