Girls 1600
Before we dive into the next group that appear ready to make a big leap forward, it's worth pointing out that most of the girls we mentioned on the 3200 page could also break through in the 1600. Runners like Emma Arriaga, Dorostkar, Wallace, and the Quintero sisters have never broken 5 minutes, and all of them have the cross country times to support them getting past that barrier this spring.
Let's turn our attention to a different set of athletes, though: these are girls with strong 3200 times whose 1600 PR's have not caught up yet. Sofia Castiglioni is a good example, with her 10:34 3200 PR bu a 5:00.1 best in the 1600. Runners with similar cross country and 3200 times are in the mid-4:50's (and even a few under 4:50), so expect her to bust through that milestone easily over the next few months. Sarah Shulze and Chloe Arriaga both go in this group, as well. Shulze's 3 Mile and 3200 times are already elite, and that seems to indicate coming inprovement in her 4:59 1600 time. Likewise for Arriaga, whose 3200 and 1600 are likely to improve (I'm predicting that she will join the sub-4:50 club this spring, putting her in the US top 50 and in the hunt for top 25). Watch out for the other Arriaga sister in this race, as well: Emma has an 800 PR of 2:14 but has only run 5:08 in the 1600. Given her improvement at 3 Miles this past fall, she has the profile of a 4:50 miler if she puts her focus there. Although she is already an established state-wide star int he 3200, Corie Smith has the profile of a sub-4:50 miler despite having a PR of 4:56, so she is an excellent candidate to move into national recognition in that event.
However, my pick for the biggest 1600 breakthrough this season is Jill Walker. Given her improvement during the fall, her PR of 5:13 (for the full mile) seems destined to be shattered. She also has the potential to drop her 3200 time significantly from her previous best of 10:42, but I think the potential for her to break 5 minutes qualifies as a bigger improvement.