What I'm proposing is keeping the current system without having to overhaul it needlessly, while changing it into something more complex and that pleases most camps of players.
First of all, from rank 30 onwards there could be "themed" skills that are unsubbable cross-class. Each current class can have up to three themes, for example a gladiator may have paladin, warrior and gladiator themed skills. Each theme (aside from the original class theme like glad) is "unlocked" by doing a quest at around rank 30 (which makes it so the existing gladiator role won't conflict with the new paladin theme for example). Not all post-30 skills need to be unsubbable however, as people could still sub the higher tiers of the more generic abilities and spells from pre-30. Ability point system can be kept as-is, although it may be beneficial to make it so the points are separated for cross-class abilities and main-class abilities. Further, unlocking each theme could give you a title that shows what your specific theme at the time is- paladin, warrior, gladiator etc. These titles could also give you perks that enhance the affinity of some of the cross-class equippable spells and skills (a paladin may have a perk that raises the affinity of support astral spells like cure).
To further enhance the system, by having a white mage themed conjurer and a paladin themed gladiator at high rank enables you to use few themed skills from either class cross-class. Each theme could have it's own "sister theme" that allows for this kind of flexibility, although with heavier restrictions.
In the end, I admit, the system may look like having talents, but really, not exactly. It's fluider, allows for more freedom and is loyal to the Final Fantasy traditions while not really stepping on the toes of the old classes that some (like me) like as-is. And best of all, SE really doesn't have to change that much in the original system, only polish and expand it. The result is different enough to feel like a new system even though the core hasn't changed much, while giving both the people who like the current classes and people who like the old classes what they want. The current classes become the "standard" classes of XIV that people start with, yet are not forced to play for longer than 30 ranks if they want to do something else, but it is still possible.
Of course, criticism is welcome and appreciated.