One of my main complaints against FFXIV has long been how we don't have customization gameplay. All paladins play the same, all bards play the same, all red mages play the same. We're all cookie cutters of each other. Is it balanced? Absolutely. FFXIV is a fine example of balance. Is it mind numbingly boring? Also absolutely.

If they added what you suggested, solo content would be insanely more fun than group content. Skill trees, subclassing, unique ways to aquire a slew of abilities and customize your character based on what abilities you use (like Blue Mage) are all a dream come true.

I would suggest screw limiting that to solo content. Give me that game, the one I'm playing now is boring.