I would put an ES inferno on a mag dk before anything else. If you had gotten an ice staff then I would think both magden and magcro could benefit. Both Colossus and Boneyard are frost damage, stack those with some ice staff skills you might have something.

If you look at the other Grave Lord skills, you also have flame and shock damage represented. One of the great applications of ES is that Force Pulse/Crushing Shock procs all three flavors of ES buffs at once so you could still use an ES flame staff on magcro and use multiple elements. In that case BTB is not a bad set to use with it. Potato s&b would work, could go maelstrom resto.