First, if all you’re really looking for are more reasons to differentiate your rotations based on other players, that much can be done through adjustments to rotations and their skills, without any need for an new system. What will likely appear to be done over all rotations is to slight de-optomize the optimal rotation, making other (albeit equally purposed and cohesive) rotations less inferior; in actuality it will be exact other rotations brought to the fore. Simply put, it will be intended emergence; you’re trying to adjust pay-offs and rotations such that composition can have an effect. Effects shared between multiple classes are a good start for this. Some of the worst for this are skills with differing priorities or varying in raid-shared to personal effects that are necessarily linked (Disembowel and Chaos Thrust); as long as Chaos Thrust retains such massive damage, this makes it very difficult for one Dragoon’s Disembowel to allow any other a different rotation; they must go through Disembowel, themselves, to reach Chaos Thrust, which they can scarcely afford to let drop, anyways. Consider how many of these jobs with such rotations or gameplay that are problematic for compositional variance have problems perceived in the same skills for different reasons. (For instance, could changes to Whirling Thrust and Fang & Claw, making them less identical and/or less rigidly timed, fix these both the compositional-independence and issues more widely perceived, without creating new issues?)
With enough such changes, you may well see that jobs individually better appeal to their players, while players are each much more interested in what others of the same job or with any shared effects are doing. This won’t have the same broad effects as a universal system, but it may well have a greater burden of knowledge and carry a greater feeling of needing to know the playstyle of your partnered characters; the fact that this direct, potentially rotation-affecting bond exists only with certain classes may also be perceived as deepened identity.
But if that’s not enough, and you really do want that new system to shake things up…
Let me append three major conditions.
1. There must be reasonable excuse and impetus for the system’s addition. Otherwise, it feels like an admitted mistake, rather than added content (in the way of systemic gameplay). This can be a theme for a new expansion, for instance, or a new (rather than a retake in) announced gameplay direction, etc.
2. If a system that would adjust gameplay is to be used universally, then the changes to universal gameplay to support it must in themselves feel beneficial and progressive.
3. If the system is to be used tactically (in the sense of choosing to save opportunities for greater opportunities), it will need a limiter; this can be through windows, a resource, limited triggering events, conflict with general rotations, or whatever else.
[I’m not saying you lack any of these conditions. This is just how my mind works, from where I start, and how I move forward. I set parameters for what is and is not good design going forward, as to reduce time spent backtracking.]
Two major design paths — coexistable and synergistic, but still separable.