While recent films have shown that Arnold is still capable of action, he might not be the guy getting his hands dirty with the likes of Superman or Captain America.
Still, there’s plenty of great roles he could play.
So much has changed in the 20 years since Batman & Robin that in addition to the fact that comic book movies have taken on at least a bit more realism, an Arnold Schwarzenegger who is 20 years older would likely be considered for very different comic book movie roles than he was back then.