Base URL issue in Subdomain
I like adding the redirect to sites/default/settings.php instead of .htaccess (just because Pantheon ignores .htaccess redirects and a lot of my sites are hosted on Pantheon). I'm also just more comfortable with the syntax.

The first example will redirect just /suggestions to example.com/suggestions. The second example redirects any path beneath /suggestions (e.g., /suggestions/submit or /suggestions/submit/thankyou) to that same path on the main domain (example.com).

In terms of wild-carding this solution for scenarios similar to 'suggestions', I don't think that would work. I mean you don't want to redirect everything so you have to check for a specific REQUEST_URI. 

But you could use this solution as a template for the other redirects you need. If you have a very large number of these (or anticipate a large number in the future), you might consider switching to the Domain Access module instead as it would handle this scenario without any redirects or custom coding.

You'll need to update 'example.com' to match your main domain and you may need to change 'http' to 'https' is you're using TLS.

// 301 Redirect from /suggestions to example.com/suggestions
if ($_SERVER['HTTP_HOST'] != 'example.com' &amp;&amp;
  $_SERVER['REQUEST_URI'] == 'suggestions' &amp;&amp;
  (php_sapi_name() != "cli")) {
    header('HTTP/1.0 301 Moved Permanently');
    header('Location: http://example.com/suggestions');
    exit();
  }

// 301 Redirect from /suggestions/* to example.com/suggestions/*
if ($_SERVER['HTTP_HOST'] != 'example.com' &amp;&amp;
    $_SERVER['REQUEST_URI'] == 'suggestions' &amp;&amp;
    (php_sapi_name() != "cli")) {
      $url = $_SERVER['REQUEST_URI'];
      $url = substr($url, 13); // stripping '/suggestions/' which is 13 chars
      header('HTTP/1.0 301 Moved Permanently');
      header('Location: http://example.com/suggestions/'. $url);
      exit();
  }