Windows.Forms.Keys to OpenTK.Input.Keys

You could make a Dictionary Map...

Dictionary<Windows.Forms.Keys, OpenTK.Input.Key> keys = new Dictionary<Windows.Forms.Keys, OpenTK.Input.Key>();
 
//Then do:
keys.Add(Windows.Forms.Keys.A, OpenTK.Input.Key.A);

Then if you want to get the OpenTK equivalent of a pressed WinForms key just do: keys[Windows.Forms.Keys.A] and it will get you OpenTK.Input.Key.A
Of course to make such a list is boring work but you just have to type it one time :)

But indeed a (static) function for OpenTK-Key to WinForms-Key might be nice, any chance/interest in having this in the library? Is "clear out all WinForms dependencies" (which I think is a really nice and want to have thing) a problem here?

From experience this is usually no good thing to do since the codes may be different on other platforms, or even change for one platform (the system is still consistent then unless something relies on the castdown to e.g. integer).