I\'ve an application which is basically a designer with preloaded controls where you can design your pages using the controls.
I\'m planning to release more and more
You should have a look at System.Addin namespace, as it really fits your need. After an addin is developped, you just have to drop it in a folder, and it is available (at runtime) for your application.
See this question for comparison : Choosing between MEF and MAF (System.AddIn)