No helmet or toggle for the purists. Putting a helmet on a character that obscures the face, makes it harder to emphasize with them because the human brain is wired to recognize facial expression. That is why mooks and Stormtroopers have helmets, because they aren't as important as the main cast Luke and his crew.
Riot knows that putting a full helm on is a bad idea if they want to push character in their new champions. (Good idea if the plan is to just have a cool character which was the old modus operandi for league)
Look at Pantheons updated splash art. They made the black area around the inside of his helmet lighter and now you can make out his facial features.
Maybe Mordekaiser will keep a helmet though. It adds to the Sauron factor. In his case they can make the helmet look metal as fuck and that will be his personality.