Hey I'm working on a 'fun' duel deck for me and a friend, and the idea is this: white vs black doppelgänger/lookalike.
Examples:
{Sanctuary Cat} = {Black cat}
{knight of glory} = {Knight of infamy}
{Mikeaus the Lunarch} = {Mikeaus the unhallowed}
More ideas?
If not limited by expansions...
{White Knight} = {Black Knight}
{Moonlight Geist} and {Searchlight Geist} work :o