Here's you purpose:
RewriteRule ^(.*).html $1.html/index.php
But I guess you can do alternatives way that will not cost you making all those directories... Or maybe you're right that's the right and the only way to do it (For your purpose).
You can't do this action because the browser will keep redirecting...
On Apache by default it's redirect /dir to /dir/ if it is available, But I guess you can disable that from the config file (It is possible on nGinx at least) But that will force you and your visitors to put the "/" after every URL in case of directory.
Our .htaccess will redirect it as a rewrite again and will never get the right path...
So instead of that you can use a .htm page by:
RewriteRule ^(.*).htm $1.html/index.php
Or (mostly) any other form you choose will work fine.