Ok. You should look at the file components/com_hikashop/views/checkout/tmpl/payment.php and get the code of the credit card form which is between the
}else{
and the
}elseif(!empty($method->custom_html)){
lines and in the onPaymentDisplay, instead of using $method->ask_cc=true; to use the standard hikashop form, you can add the creditcard.js file, and set in the variable $method->custom_html the content of the form from the hikashop view. That way, you can have your own HTML form in your plugin. You can also retrieve the additional information in that same function and store it in the session for later use in the onBeforeOrderCreate function.
That way, people won't have to edit the default view so that it works with your plugin.