Most apparel may not be customized by rule. Famously, Earl Bennett is very committed to his bright orange shoes this year. He has paid $15k in fines so far to wear orange shoes, which is a violation of rules about appearance. Some alterations may be done "under the skin" such as flak jackets. Some players have custom padding under the helmets. You will see a huge influx of technology into padding and especially helmets in the next few years. I think within 4 years you see helmets with capability to monitor heartrate, temperature, etc. All that being said, you will always see players with the bridge of their nose and foreheads bleeding or red from impacts and tight fitting helmets.