I don't subscribe to the "Marik is important/happening" club, but I suspect his signature card would be Metal Reflect Slime if anything. Ignoring Mokuba (for obvious reasons), no character so far has had a signature card that wasn't used by them in the anime. Revival Jam probably wouldn't be even though it probably deserves that slot more, and MRS isn't in Duel Links yet, so gives him a drop.