I compiled a list of all of the type combinations that haven't been used yet. These include both Alolan forms and Mega-Evolutions. I'm pretty sure I got all of them.
- Ice/Normal
- Fire/Grass
- Fire/Ice
- Fire/Fairy
- Normal/Poison
- Normal/Rock
- Normal/Bug
- Normal/Ghost
- Normal/Steel
- Electric/Dark
- Electric/Fighting
- Electric/Poison
- Psychic/Poison
- Psychic/Bug
- Ice/Poison
- Ice/Bug
- Dragon/Bug
- Dark/Fairy
- Dark/Bug
- Fairy/Fighting
- Fairy/Poison
- Fairy/Ground
- Ground/Fighting
- Steel/Poison
- Ghost/Rock
I'd include
Electric/Grass, given it is currently only a Rotom form typing. I'd also include
Ice/Electric and
Electric/Fire for that same reason.
I'd love to see evolution lines for all three. I'm not sure what all could be done for Ice/Electric, but I can easily see all kinds of mythical creatures being used for Electric/Fire. I'd particularly like to see an evolution line based on the Nigerian Thunder & Lightning myth (the beast Lightning running around causing fires while his mother Thunder shouts at him).
As for Electric/Grass, I'm sure they can just play around with the fact that many plants are capable of conducting electricity, even though in small amounts. For the simplest and most relative concept, there could be a pokemon based on the lemon battery. For a more complex but much more rewarding concept, there could be a pokemon based on the concept of 'tree power,' the practice of harvesting electricity from trees.
As for Ice/Electric, the one concept I can think of is the science behind thundersnow, the occurrence of lightning during a snowstorm. The only way this could work for a pokemon, though, is if there is a lightning-related mythical creature or electricity-related animal that exists in a relatively cold country.