How does CoCS limit design space? In my opinion one just has to get a little more creative when balancing new skills.
Let's look at an example of a slow, hard-hitting spell that is intended for self-casting purposes:
As it stands, the low cast time is completely ignored by CoCS, and so the spell would be magnitudes stronger on CoCS compared to self-cast.
If that spell now had a cooldown significant enough to prevent it from being useful for CoCS, chances are that self-casters also don't like the slow (because limited) cast rate.
To remedy that situation, what if we had cast speed affect spell cooldowns? (Or, for that matter, introduce a new stat called "spell cooldown reduction" that appears in combination with cast speed passives - then it doesn't get as ridiculous with items like Pledge of Hands and is easier to balance.)
I'm quite optimistic that the average CoCS-character has no passive points left to also include cast speed in their tree. Also cast speed on gear is no option (due to having limited options in the first place) and frenzy charges - well, which passives to sacrifice to get more than 3 of them?
And regarding the potency of spells casted by CoCS versus self-cast:
I'd guess* that the self-caster (if similarly offensively build) has an advantage of about 300-400% increased spell damage over the CoCS character, while still having access to all the same spell crit nodes.
*meaning this is an assumption - I did not research this numerical range