[Bruno]

You can't just make a solid plate out of transparent whatever, unless you're going to add a life support system. There is definitely not enough air circulation with a helmet-like bucket on your head to sustain athletic activity, and the way fantasy PCs tend to wear their armor 24/7, I'd even be worried about surviving the night.
You'd also want climate control, but by the time you're making invisible helmets a Cool enchantment should be manageable.