Hm. Maybe this set will bring back dual types to the TCG, based on the names of the LEGEND cards. For example, the Raikou part of the card would be a {L} type, and the Entei part would be {R} type. Until we see pictures, though, we can only speculate.
Now that you mention it, it is very odd how there were no Darkness or Metal types in HeartGold Collection or SoulSilver Collection. I mean, G/S introduced them.