Can leukopenia in a person with an otherwise normal immune system lead to non-infectious diseases in the long-term?
Leukopenia is just a definition. In medicine, there's a saying:


  Treat the patient, not the numbers.


Numbers can mislead people, sometimes quite seriously. Definitions of leukopenia vary: some define it as less than 5,000 WBCs per µL of blood; others define it as less than 4,000.

Decreases in specific WBCs is usually described as neutropenia, lymphocytopenia, monocytopenia, etc. This, too, is important information, as each cell has a different function, and thus different results are possible.

In leukopenia, bone marrow biopsies are very informative. They reveal if the cause is a decreased production of WBCs due to destruction of precursor cells, replacement of marrow by other tissue, invasion of marrow by cancer, etc. If a bone marrow biopsy is normal, that is very comforting.


  does the condition need to be treated?


Most people with anti-epileptic drug (AED) induced leukopenia are just fine. Some common-sense precautions (such as avoiding other drugs which might also cause leukopenia, and checking a white count if you fall significantly ill) should be heeded. If your doctor is a neurologist, they should be thoroughly versed in the side effects of AEDs, as this constitutes a very large patient population for them. Some of the newer AEDs have a lower incidence of leukopenia. Talk to your doctor about the potential benefits and drawbacks of switching.


  Does leukopenia increase a person's chances of getting some non-infectious diseases too?


Theoretically possible, but not particularly likely.

If you are truly concerned, ask if you can get a referral to see a hematologist. They should be able to give you a real picture of the problems with long-term AED-induced leukopenia.

Remember: treat the patient, not the numbers. If you're healthy now, you're likely to continue to be so. Maybe you'll need more blood tests than others, but that doesn't mean you're not healthy. Talk to your doctors, not the internet.