Bug/Dark, Bug/Dragon, Bug/Ice, Bug/Normal, Bug/Psychic
Dark/Electric, Dark/Normal, Dark/Psychic
Dragon/Fight, Dragon/Grass, Dragon/Poison, Dragon/Rock
Electric/Fighting, Electric/Normal, Electric/Poison, Electric/Psychic, Electric/Rock
Fight/Fly, Fight/Ghost, Fight/Ground, Fight/Ice
Fire/Grass, Fire/Ice, Fire/Normal, Fire/Poison, Fire/Water
Ghost/Grass, Ghost/Normal, Ghost/Psychic, Ghost/Rock, Ghost/Steel
Ice/Normal, Ice/Poison, Ice/Rock, Ice/Steel,
Normal/Poison, Normal/Rock, Normal/Steel, Normal/Ground
Poison/Psychic, Poison/Rock, Poison/Steel
I may have missed one or two, but thats most of the 40 or so combos still missing. Poison is the most unrepresented, still missing 7 subtypings (not that poison can hurt much or provides much immunity) while being super commonly paired with grass. Dragon/Poison, Electric/Poison, Fire/Poison, Ice/Poison, Psychic/Poison, Rock/Poison, and Steel/Poison. Of those, Psychic/Poison and Steel/Poison are probably the most interesting since Poison is weak to Psychic and Steel is impervious to Poison.
Electric is pretty shafted too… if you aren't counting Rotom's alternate forms, its still missing 7 types. But since they've made it easier to access Rotom, and its different forms, its only missing four.
Ice, Fighting, Ghost, Psychic, and Rock are all missing five apiece. Including ghost/psychic, which would be interesting since they'd balance out weaknesses... but be doubly weak to dark.
Fly has the least left, with only Fly/Fighting left.
Normal subs are odd since they don't add much... since they only provide an non-effective stab and a weakness to fighting. But I guess they add in the immunity to ghost still?
Fire/Grass, Fire/Ice, Fire/Poison, Fire/Water. Three of those pair Fire with an opposing type and are therefore of immediate interest. Opposing type combinations are always great since it automatically knocks out a weakness — Fire/Grass, despite being part Grass, would obviously not be weak to Fire. It would also not be weak to Ground, despite being part Fire.