Pokémon Sword & Shield have added a fair few new Pokémon to the series, but have also cut almost half of the original Pokédex in the process. There are new Pokémon in the Galar region that have unique types, such as the Impidimp line being Dark/Fairy and Galarian Weezing being Poison/Fairy.
There are almost 900 Pokémon in existence, yet there are still many type combinations that have yet to be used in the series. In some cases, this is due to potential balancing issues for Pokémon that could be too strong in terms of resistances, while others are more mysterious and could potentially be a result of Game Freak specifically holding back ideas to use in future games. There are still sixteen unique type combinations that have yet to appear in the Pokémon series, even taking into account the recent additions from the Galar region.
Normal + Ice/Poison/Bug/Rock/Ghost/Steel
Normal is the most common Pokémon type in the series, yet it still hasn't been paired up with several other types.
A Normal/Ice-type Pokémon would likely be decimated in competitive battles, due to a X4 weakness to Fighting-type moves and X2 weakness to common Rock, Steel, and Fire-type threats. A Normal/Rock-type Pokémon would also suffer on the battlefield, thanks to a X4 weakness to Fighting-type moves and X2 weakness to Ground, Steel, Water, and Grass-type moves.
It's easy to see why a Normal/Ghost-type wouldn't work, as it would possess three total immunities to Normal, Fighting, and Ghost-type attacks, while only taking double damage from Dark-type moves. A Normal/Poison-type Pokémon would also fair well, due to only taking double damage from Ground and Psychic-type moves, while resisting Poison, Bug, Grass, and Fairy. A theoretical Normal/Steel-type Pokémon would also be OP, due to how it would resist ten different types of damage and be immune to two.
It seems strange that a Normal/Bug-type Pokémon has yet to exist, due to how common the types are and how it would have balanced weaknesses.
Electric/Fighting
The fact that there isn't an Electric/Fighting-type Pokémon on the list might be the most surprising omission of all, considering how these two types would seemingly go well together in terms of concept. It's easy to imagine a martial arts-themed Pokémon that powers up its attacks with electricity in order to shock opponents while punching/kicking them.
The type matchups for an Electric/Fighting-type Pokémon are also fairly balanced, with it resisting five types (Rock, Bug, Steel, Electric, and Dark), while taking double damage from Ground, Psychic, and Fairy. It would have no immunities or X4 weaknesses.
Ice/Poison
It's easy to imagine an Ice/Poison-type Pokémon, but the reason one doesn't exist yet might have to do with status effect immunities. An Ice/Poison-type Pokémon would be immune to both freezing and poisoning (except in exceptional circumstances), which would protect it from some of the most common status effects in the game. The resistances and weaknesses would otherwise be fairly balanced.
Fighting/Ground
There has been a Rock/Fighting-type Pokémon in the past in the form of Terrakion, but there has yet to be a Fighting/Ground-type Pokémon. A theoretical Fighting/Ground-type Pokémon would have some severe strengths and weaknesses, in the form of taking double damage from six different types of damage, while resisting three types, taking quarter damage from Rock-type moves, and being immune to Electric-type moves.
Poison +Psychic/Steel
A theoretical Poison/Psychic-type Pokémon would be fairly balanced, with three double weaknesses, three resistances, and one-quarter resistance to Fighting-type moves, so it's odd that there are no Pokémon of this type in the series yet.
It's easier to understand why a Poison/Steel-type would not exist, due to the fact that it would have six resistances, a one-quarter resistance to Bug, Grass, and Fairy-type moves, and immunity to Poison-type attacks. This would be slightly offset by a X4 weakness to Ground-type moves, but the Poison/Steel-type Pokémon would have the advantage in lots of different situations.
Ground/Fairy
A Ground/Fairy-type Pokémon would also be pretty beefy in terms of its defenses. It would have total immunity to both Electric and Dragon-type attacks while resisting four others. It would have X2 weakness to four common types of attacks (Steel, Water, Grass, and Ice), but it could still be quite a threat to certain teams.
Bug +Dark/Dragon
It's strange that there hasn't been a Bug/Dragon-type Pokémon to date, considering that it would make sense for there to be a Pokémon based on a dragonfly that has draconic qualities. There is a dragonfly Pokémon in the form of Yanma and its evolution, but those are Bug/Flying-type Pokémon. A Bug/Dragon-type Pokémon would be fairly balanced in terms of its types, so it's likely that one will appear in the future.
A Bug/Dark-type Pokémon also seems fairly likely, due to how balanced it would be in terms of typing and how easy it would be to come up with a concept. There are a lot of insects in the world who live in dark, isolated places that could easily be reworked into a Pokémon.
Rock/Ghost
There have been several Ground/Ghost-type Pokémon in the form of the Golurk and Sandygast lines, but there have yet to be any Rock/Ghost-type Pokémon. The Golurk and Sandygast could easily have been changed to Rock/Ghost-types and still maintained their concepts (as is often the case with Rock and Ground-type Pokémon) and the typing is actually more balanced than Ground/Ghost in terms of weaknesses, so maybe this will also appear in the future.