A variation upon a trope or story mechanic which, like an isotope of an atom, remains fundamentally the same as the normal version, but may behave very differently.
Although dracula and edward are both vampire isotropes, the dracula isotrope is highly stable, while the edward isotrope has a tendency to decay into fanfiction and ghey
IsoTope may indeed have had just a minor role in the "Mystery Of Edwin Drood" tale, but I am still a bit surprised that neither Dickens nor Garfield made any mention of him in the famous mystery-thriller story.