Lots of idea shooting here.
Customization of classes isn't something new to MMOs. It is completely doable and probably should be done. It's also completely doable with the current system in tact. I'd be more interested in seeing some classes opened up for multiple roles honestly. The low availability of tanks in this game is a tremendous short sight that should have been fixed months ago. Much of that has to do with the fact that there are only two classes that can tank which is really kind of stupid. There are a couple of other classes currently that with a little tweaking or a customization/talent system could also tank. Ninjas were awesome evasion tanks in FF11 for a while and I'm pretty sure they were never really intended to be but the players decided they could do it. Why? because they were given that freedom. Dragoons here can wear heavier armor and have some decent defensive capabilities but here they are pigeonholed into just another face among the many dps classes.
The issue is a lack of freedom in character development. Of course people will theorycraft and number crunch to min/max anything but that doesn't mean it needs to thwart giving us options. Because those options don't necessarily need to mean "Oh I still want to be a DPS/Healer/Tank but you know... different."
However the problem with threads like this is most people think so narrowly that they already have their minds made up long before they read any other posts, and because of this they're not able to think innovatively. And this game sorely needs some innovation.