I think the 400/400H double is most likely. Her only real rival in the 400H, Femke Bol of the Netherlands, is moving up to the 800. One obvious reason is that she knows it is unlikely she will ever beat Sydney in a major competition (barring an injury or some other bizarre occurrence.
The 400 is an interesting event because it can be run by sprinters who are moving up a distance or 800 meter runners who are moving down. I always felt like Bol was actually more of an 800 type who had found success in the 400H. So her moving up is probably playing to her natural proclivitie.
Sydney McLaughlin-Levrone seems more like a pure sprinter who has moved up.