Showing some content instead of the posts

Won’t the ‘the_content’ filter hook do what you need?

From the Codex:
https://codex.www.ads-software.com/Plugin_API#Current_hooks_for_filters

the_content – applied to post content prior to rendering. Passed the content as a string.

and then just prepend your HTML to $content?

function foo_bar ($content = ”) {
return $your_custom_html . $content;
}

add_filter(‘the_content’, ‘foo_bar’);

Or did you want to do something different?

Thanks a million!
This was exactly what I was looking for.
Cheers,
Davide