Husserl suggested an intriguing notion of passive synthesis, which I believe in his view would be the source of what Hegel calls mediated immediacy. Something like this could help round out the Kantian account of synthesis, which is strongly tilted toward active synthesis associated with deliberate conscious acts. Language is straining here; passive synthesis is just short of paradox. I think the idea here is that “passive” synthesis is synthesis that comes to us more or less ready-made, based on previous syntheses we have formed or socially assimilated.
Something related to this is also involved in the Aristotelian notion of a “common” sense, which in effect synthesizes our experience of sensible things into a unified whole. Aristotle’s remarks are in a very minimalist style, but were developed in a little more detail by the commentator Alexander of Aphrodisias. Kant and Aristotle both generally treat sense perception in passive terms, but Aristotle’s common sense clearly cannot be entirely passive. Many medieval authors working in the broadly Aristotelian tradition developed more detailed accounts of sense perception in general as also involving an active component.
These various efforts to describe such processes in terms that are neither purely active nor purely passive seem very important to me as laying the ground for a reasonable account of human agency, free of both voluntarism and simplistic determinism.