I think it would be interesting if it was another Dragon/Steel type, because it seems logical that the 3rd game will be called "Pokemon Grey", and its mascot could also be grey, (so it is a mixture of black and white, and grey/silver is the colour of steel). Also, steel is weak to fire and electric (Reshiram's and Zekrom's secondary types, respectively), so it could be Dragon/Steel.
However, I agree with what most people are saying about it probably being Dragon/Ice, because we have seen a number of trios have the Electric/Fire/Ice types (i.e. the legendary birds, Jynx, Electabuzz and Magmar and the legendary dogs), so it seems highly likely that the 3rd mascot of the 5th generation will be Dragon/Ice. If that's true I would love to see what it looks like! :3