I believe these are all the type combos that have never been done or aren't yet confirmed to be in SM. Which would you like to see done in the future?

Normal/ Poison
Normal/Rock
Normal/Bug
Normal/Ghost
Normal/ Steel
Normal/Ice
Fighting/Ground
Fighting/Ghost
Fighting/Electric
Fighting/Ice
Fighting/Fairy
Poison/Rock
Poison/Steel
Poison/Electric
Poison/Psychic
Poison/Ice
Poison/Fairy
Ground/Fairy
Rock/Ghost
Rock/Electric
Bug/Water*
Bug/Psychic
Bug/Ice
Bug/Dragon
Bug/Dark
Fire/Grass
Fire/Ice
Fire/Fairy
Electric/Dark
Dark/Fairy

*Surskit and Wimpod are in fact Bug/Water, but there are no fully evolved mons of this type. Holding out for Champod.