How about an additional essence each time the Charisma bonus is doubled (1 at 1, 2 at 2, 3 at 4. 4 at 8...)? At 40+, it will give six essences, looks quite managable.
That could work I guess. I was also thinking of limiting that to a number of equivelent spell levels of shapes/essences = to Charisma bonus. I don't know if that would work or not since they're so low level.
On a similar note, does anyone think Immortal Warlocks need new Shape/Essence Invocations? I would think Eldritch Empowerment (as written) probably negates the need for any more.