Question about $config['url_suffix']

You can also create an htaccess rule and redirect any url that doesn't have the .html extension to the same url but add the .html extension.

That is a good idea. I'll try to find a rewrite rule to do that. CI have the ability to use only allowed characters ($config['permitted_uri_chars']) maybe in the future CI should use a function like that to verify the suffix to enforce it and make a 301 redirect if the suffix is in use and not present in the uri string. That way people don't have to deal with rewrite rules.