Jeremy Felt

In some cases, it’s necessary to provide a summary of each post in your primary RSS feed while still syndicating 
your content with full text to various parties. Dual RSS Feed Key allows you to enter a secret key to use with 
your RSS feed to display a full text version of a site’s feed to those who have the key.

Dual RSS Feed Key

Provide a "secret" full text RSS feed on WordPress sites with summary RSS feeds enabled.

MailerLite

This plugin automatically adds featured images of your posts into the RSS feed.
Features:
<ul>
<li>Automatically adds featured images of your posts into the RSS feed</li>
<li>Choose from all available image sizes</li>
<li>Choose from multiple image alignment options</li>
</ul>

Featured Image in RSS Feed by MailerLite

This plugin automatically adds featured images of your posts into the RSS feed.

Dual RSS Feed Key vs Featured Image in RSS Feed by MailerLite