Ability Power at least would increase the Shield Value.
Depending how Karma is played; I find myself buying
or
only in kinda rare cases.
I prefer to build her more AP as Support because as you said Karma is more effective with some AP.
The Problem with a Passive that brings Damage only to Karma is that it will overload especially her Q with Damage and forcing her even more in RQ's than it is now.
So what is the goal of your proposal?
Do you feel her Passive is not good enough? Do you think Karma does not deal enough Damage?