You're absolutely right in that current caster classes lack a "theme" as it applies to combat gameplay. However, I don't think that can be solved in a system like this alone. I also have a huge amount of respect for solo combos. And having also played a WoW mage (other favorite classes being warrior, hunter, and death knight) I know exactly what you mean there.
In the end, solo rotational strategy is about having a critical balance between what can be changed, and what can't. We push our limits to certain tiers (for a long time I was in love with the WotLK Improved Steady Shot gamble via haste using proc-trinks in a precisely manipulable fashion) in order to reach a new tactical difference (being able to fire off exactly 3 ISSs before my next CD ability, or exactly 4 given proc one or 5 given double proc, etc).
Though, that might only be true for setting oneself up in preparation for a fight. [Additionally, it's very doable without ability cooldowns, using a more analog system of gliding strength per ability and sets of abilities instead.] Either way though, it needs to stem off something larger, more intricate. WotLK's talent systems were a good example of this. Whether you like the idea of the areas of choice being identical for every player or not (I personally don't), they had sufficient complexity and enough unpredictable uses to make some truly unique playstyles. FFXIV would first need something similar. That in turn requires a new layer of depth in leveling, where FFXIV again falls short.
Though this system could add far more strategy to casting, its ideal uses are only going to come in combination with larger changes, the majority of which are probably overdue.
That said, I would never try to copy WoW's system. WoW's talents system (paired with its abilities) worked for WoW because it was WoW, not just because it's a universally powerful system. What I'd make for FFXIV would stem from Final Fantasy instead, the more fundamental or thematic the connection the better. Mechanics are made to serve a story and to serve play. Not the other way around.
Not saying that's what you seemed to be hoping for at all. I agree with everything you've said here. The last rant was only what I can call a "head's up".
In summary: Those issues are beyond the domain of this system alone.