Splitting the plugin into several filesSolved

Your plugin (oxide/plugins/Example.cs):

using Oxide.Ext.ExampleExt;

namespace Oxide.Plugins
{
    [Info("Example", "Me", "0.0.1")]
    public class Example: CovalencePlugin
    {
        private Init()
        {
            LogWarning(GetHello());
        }
    }
}

Your include file (oxide/plugins/include/Ext.Example.cs):

namespace Oxide.Ext.ExampleExt
{
    public class ExampleExt
    {
        public string GetHello()
        {
            return "Hello world";
        }
    }
}

This is based on memory, so may not entirely be working. I believe // Include is the actual magic comment to use, not // Requires like I originally mentioned in the thread you linked.

Unfortunately, this functionality is limited. I want all plugins to have a reference to my requests, because it will not differ

Merged post

Roughly speaking, the library

This would be the same functionality as referencing native C# files essentially. The only other option is writing an extension or standard DLL.

Well, in the DLL example, how do you plug it in?

rtbbebd

Well, in the DLL example, how do you plug it in?

Any DLL you want to use would have to be referenced by your plugin. For extensions, those are loaded automatically by Oxide. For regular DLLs, you'd have to add a "// Reference: DLLName" to the top of your plugin in order for the DLL to be picked up by the compiler.