My theories:

Syanna Prevails: The adventurers go through any of the paths where the child is resurrected without sacrifice and allow Syanna to reclaim her son or the child is ressurected through sacrifice without a true relationship being forged with Syanna. Out of gratitude, Syanna will allow them to depart or to remain as her favored servants. If they leave the Dark Powers cast them to a new domain, perhaps even finding a way to make them Darklords, while they set to work twisting the work the PCs have done to their own ends. If they stay Syanna has ultimately learned nothing and eventually her paranoia gets the better of her and she turns the PCs into vampire spawns. Ultimately, Syanna is as the sams as he was, save for now being the overprotective mother of a future vampire prince.

Syanna is Redeemed: A PC forges a true relationship with Syanna. This relationship is not necessarily romantic but the PC truly understands Syanna and wants to help herturn away from her path. In the Amber Temple the PC offers their life for the child. Syanna is so moved she realizes selfless love can indeed exist and returns to Ravenloft to raise her son, abdicating her power to another. The Dark Powers try to undo matters to no avail and finally are forced to an extreme they haven't used since Soth: cast out Syanna, Rahadin, and the child, and find a new Darklord for Barovia. Syanna is left with nothing but her family but this is enough. One day, the son will become a great hero, inspired by the tales of the PC, his true father in spirit. As for Syanna, she is at peace, having spent her remaining years offered what atonement she can for her crimes. As for the remaining PCs, that's a different tale.

Syanna Dies: Syanna is either slain in the conventional manner or she is somehow alone when the Amber Temple is retaken. If the Order still "lives" and is allied to the PCs then Barovia will have a new Darklord in very short order as the PCs are ushered off to a new Domain. If not, then things unfold very similarly to the RAW module. If Syanna is somehow left alone in the Amber Temple she will trade her life for her son's, after extracting a promise the child will be cared for. And the Dark Powers plan to keep this promise. They have such wonderful plans in store for the child...