Serena is going to go for the jugular. She already has access to Hannah and the Mackenzies, and her stay in Gilead is believed to be temporary, so it won't take long to figure out what she plans on doing. In the season's second trailer, we see Hannah walking in a line with a group of girls who are all wearing white. They're in a room that looks like it's part of the red center, but the beds have white lace tents attached to them.
Gilead's color system might be confusing at first, but we know what white means. Eden was wearing white when she married Nick.
Hannah could be getting married. She could also be facing a suitor or a suicide attempt. Those three fit best in the context.
Serena could try to arrange a match with Commander Judd, who compulsively kills the young girls he takes on as wives. He has an entire menu of poisons he chooses based on his whims.
In the Testaments, Hannah's stepmother tries to force her to marry Commander Judd and Lydia intervenes, blackmailing Mrs. Mackenzie into allowing Hannah to become an aunt.
This feels like the most likely scenario. Lydia is going through changes in her beliefs. In the episode 3 summary, it says that she will start to question her harsh treatment of the girls. June is going back into Gilead. Maybe the two make a deal and Lydia agrees to help Hannah. Perhaps Serena is the one to arrange the match, and June finds out and approaches Lydia.
It's hard to say, and there are a lot of options. Those are just the ones that stand out given what we know of The Testaments. The only certainty is that Serena is gunning for Hannah, and whatever happens is going to set the stage for the upcoming spin-off.