Good afternoon. I'm surprised that so far no one has suggested using modifiers in the engine. I explain that modifiers are a kind of modules that allow you to make changes to the functionality or appearance without affecting the original files. This ensures the immutability of the engine files, but at the same time you can change or add functionality. Similar modifiers can be found in many cms, I still know the opencart and dle. Sincerely...
P.S. Sorry for my english

@Hard
Basically from theme point of view, you can use child themes. In regards to plugin files, this is not possible right now.

@Hard
Basically from theme point of view, you can use child themes. In regards to plugin files, this is not possible right now.

What exactly is impossible? Indicate the path to the file and write which support to perform and in what place? For example:
<?xml version="1.0" encoding="UTF-8"?>
<modification>
 <id>Example of the vQmod</id>
 <version>1.0</version>
 <vqmver>2.X</vqmver>
 <author>Tuts+</author>
 <file name="targetfile.php">
 <operation info="Example of the vQmod">
 <search position="replace"><![CDATA[
 I am original content.
 ]]></search>
 <add><![CDATA[
 I am replaced content!!
 ]]></add>
 </operation>
 </file>
</modification>


@Hard
Sure, let's do it and let us know once ready for testing.

VQMod for osclass. I do know if it is fully working. Lets try.

https://github.com/codexilab/osclass-vqmod

Well, looks like it is developed by this person, but looking on his work,it seems he just copy work of others.

Anyway, you can download plugin and work with it