As you seem to know, the DNP is the academic degree ... the NP is a certification. Currently, most NP's are educated at the MSN level, but that will gradually change over time as more DNP graduates take the NP jobs. If you are interested in being an NP, then the DNP makes sense as it will most likely become the predominant degree in the future -- and may become the only degree for new NP's very soon.
So ... if you want to work as an NP, then a DNP makes sense for you.
If you want some other sort of advance practice role or leadership role, a DNP may also make sense -- but things are quite so clear. The DNP degree was created with NP roles in mind, but many individual programs are not encompassing other roles, such as administration, Clinical Nurse Specialist, Midwife, and Nurse Anesthetist. You would have to look into specific programs to find which "tracks" are available at each specific school.
Yes, you do need to identify your specialty area before getting too deep into a DNP program (or any other graduate level program) as your specialty determines the focus of your graduate level studies. Different schools have different requirements and offer different options. So you'll need to investigate specific schools and make your choice.
I have a PhD and work in a hospital doing Nursing Professional Development and a little research.