Pretty much you can do most of the Applied Physics. But it can be intermediating for you to go for theoretical physics since it required comprehensive physical ideas.

Now some of the best topics which have equal scope for Theorists as well as Experimentalist/ Engineering Physicist are provided below:

• ASTROPHYSICS, FUSION AND PLASMA PHYSICS (Mainly Plasma physics and the developments of ITER. Astrophysics have most application of Plasma Physics. SO you will have a chance to work in the applied portions of Astrophysics).
• NANOSCIENCE AND NANO TECHNOLOGY
• CONDENSED MATTER AND MATERIAL PHYSICS
• ENERGY SYSTEMS
• MICROFLUID AND MICROSYSTEMS
• OPTICAL PHYSICS AND QUANTUM INFORMATION SCIENCES

All this fields are the emerging fields of physics which you can study and do research and you will just get fascinated by the subject itself.

I hope this answer helps. All the very best for your future endeavors!