My interest in cardiology edged out neurology narrowly when applying to residency. By the time I lost interest in cardiology and realized more and more and more I should have gone neurology, it was way too late in the application process. The only other IM subspecialty I though was slightly appealing was ID, which I have less interest in and pays less than neuro anyways. I originally framed it in my mind that I would be applying to Neuro as if it was a fellowship, eventually do general neuro, and still be doing something I like more and pays more than ID.
But I like the overlap idea. I had no idea that NCC took from IM. That's pretty exciting news to me.
In neurology, there are two different ways to hang your hat onto a certain subspecialty. The ABPN has board exams for a number of subpsecialites (neurophysiology, sleep, vascular, pain, etc). In order to be eligible for those examinations, you of course need your primary board certification in general neurology.
The other pathway is the United Council for Neurological Subspecialties (UCNS). This organization offers certification programs in a number of neurological subspecialites not offered by the ABPN such as neurocritical care, behavioral neurology, headache medicine, etc. The UCNS is NOT a member of the American Board of Medical Specialities, thus, you cannot say "I am board certified" but to say that you earned a certificat from the UCNS is something. To clarify this, "board certified" has different meanings and in TX (where I practice) you can only legally say that you are board certified in a specialty if and only if the certification comes from an ABMS organization.
Okay, that being stated, I would encourage you to develop your interests and go for the long haul. What happens if there is no board for a certain subspecialty? What happens if there is no certifying organization for a subspecialty? E.g. you mentioned an interest in neuroinfectious disease.
Neuroinfectious disease is a real specialty and a few I know that hang there hat on that specialty of course work at major university centers and do major work/research in neurological complications of AIDS/HIV, Syphillis, and weird infections that the average Joe does not see. There is no hard set fellowships out there in neuroinfectious disease per se. Johns Hopkins has one that is combined with neuroimmunology. NIH may fund such a program through their heavy research grants as another example.
For the ones that I know who hang their hat on neuroinfectious disease, they did one of the following: double residency + IM ID fellowship (ouch!!). Or what I would deem a more reasonable option, finish general neuro residency and get an MPH on the side that focuses on infectious disease and get into researching all of those weird neuroinfectious things.
I would suggest considering the later if neuroinfectious disease is truly an interest of yours. If you finished 3 years of general neuro, then got an MPH on the side, that would perhaps be less painful than doing another 3 years of ID fellowship. But if you established yourself as an "expert" through your research, you can hang your hat. Of course, in my opinion, neuroinfectious disease is something to consider only if you want an academic career working in a major university.
Because neuroID is in its infancy, should the ABPN develop a board certification program, or the UCNS develop a certification program, there would be a grandfathering practice track phase and you would easily be able to do so.
You also mentioned a prior interest in cardiology. The NIH has a neurocardiology program. Well, that is bascially for all of those "passing out" patients, blah!