I really like a lot of the concepts OP came up with. I think that this is certainly something we could see making a resurgence in this game. Yes, everyone knows Yoshi-P said no. Genuinely asking here: Why did he say no? Are those issues still in the game? I really can't think of what problems are currently caused by the SMN/SCH split, other than the fact that someone could queue up as a healer in higher level content without having a single idea of what to do... but um, people kind of do that regardless, and jump potions are now a thing.
When it comes to gear/weapon sharing, I really can't see why, say, if my BRD had a healing spec, the healing potency couldn't scale off of DEX. And I agree what the person above was saying. Giving everyone a "free" support spec would improve the tank/heals population, and it would possibly give everyone a better awareness and understanding of different roles, thus having a positive effect on player skill overall.
And of course, they wouldn't release these alternate paths all in one patch. But I can see at least one happening in the future. As it stands, though, I think there are more unique jobs to explore.
