After spending hours trying to understand why my existing plugins were NOT loading, i've found out that there is a NEW PROPERTY on the DESCRIPTION text file that comes with each Plugin.

If NO value is found, the source code assumes it's v2.0
therefore automatically NOT loading them because it does NOT match the current version.

>> Nope.Core.Plugins.PluginManager
Line 460
            //nopCommerce 2.00 didn't have 'SupportedVersions' parameter
            //so let's set it to "2.00"
            if (descriptor.SupportedVersions.Count == 0)
                descriptor.SupportedVersions.Add("2.00");


???
I wonder why the assignment is NOT to the latest version
???


==========================================
SOLUTION
Add the following entry on the Description.text file on Solution\Plugins\YOUR FOLDER PLUGIN\Description.text
    SupportedVersions : 2.10

...or to whatever version you will be working on at the time you will read this post.