Hi,
In your plugin, you can add a function pluginConfigDisplay($optionType, $extraOption, $type, $paramsType, $key, $element, $options) and return the HTML you want. That way, you can create new type of options for the $pluginConfig attribute at the beginning of your plugin. When the type you set there is not known, the system will call your pluginConfigDisplay function so that it can return the HTML you want to be displayed.
And you can add another function called onPaymentConfigurationSave(&$element) to be called when the payment method is being saved so that you can take the data from the POST and add it in the object $element->payment_params as extra attributes or in another table of your plugin or eslewhere.