What I'd suggest is looking at a good spice companies blend. You could try mimic it or even purchase it. Here is Penzey's spices fajita spice blend.
Here are the ingredients:
Hand-mixed from: salt, black pepper, paprika, Turkish oregano, cayenne pepper, garlic, celery, Mexican oregano, basil, nutmeg, cumin, marjoram, thyme and rosemary.