First off, congratulations on the master's degree! Is your dissertation publicly available? I'm sure people here would be interested in the subject matter.
When it comes to novel research directions, it's useful to identify the unique aspects of Swift and see what they may enable that other popular languages might struggle with. Swift provides language-integrated differentiable programming (pending the Evolution process) in an ahead-of-time compiled, fast, and strongly typed language. What are applications that the host language has a significant impact on, and for which do Swift's capabilities solve outstanding problems?
Differentiating through simulations of various types is an area that I think is just starting to grow in importance. You may want to optimize parameters in a physics simulation, or use gradient descent to find cases where desired behavior emerges in an artificial life environment.
Reinforcement learning is another field where a fast host language has been demonstrated to help in many applications. That could be in accelerating simulator environments and wrappers, a known performance bottleneck. Or it could be in combining advanced logic with neural networks (AlphaGo and descendents as one example).
These are just a few of the areas that come to mind. Part of the difficulty is that current tools make some things impractical, so those areas haven't been explored and therefore aren't at the forefront of our awareness. That's why it's useful to keep in mind where differentiable Swift differs from what is widely used now, and be open to looking for problems that could solve.