Dungeons are easy to heal or tank. If Alexander doesn't require a pure healer to keep everyone alive no one's gonna want to play with me when they can get someone who can heal and do more damage and viceversa. However you make it someone's gonna be left out in favour of more "optimal" builds.

Your less extreme options sounds like you just want more cross-class skills which is what may happen already in 3.0 as more jobs are introduced. But if you want to break out of your role, you have to balance everything around any possible combinations, or using duty finder will be impossible. Dungeons are designed so that any two dps classes can pass all checks. What happens if you get into a dungeon and your dps and tanks decided to keep their tanking/dps skills at a minimum and throw everything else into healing?