Just out of pure interest, I compiled a list of Pokémon type combinations we haven't seen yet in any game. Hopefully some of this is interesting to you all, for speculation and whatnot.
BUG/NRM
BUG/FRE
BUG/ELC
BUG/ICE
BUG/PSY
BUG/DRG
BUG/DRK
DRK/NRM
DRK/ELC
DRK/FTG
DRK/GRD
DRK/PSY
DRK/DRG
DRK/STL
DRG/NRM
DRG/FRE
DRG/ELC
DRG/GRS
DRG/ICE
DRG/FTG
DRG/PSN
DRG/BUG
DRG/RCK
DRG/DRK
ELC/NRM
ELC/FRE
ELC/GRS
ELC/ICE
ELC/FTG
ELC/PSN
ELC/GRD
ELC/PSY
ELC/RCK
FTG/NRM
FTG/ICE
FTG/GRD
FTG/FLY
FTG/RCK
FTG/GHO
FRE/NRM
FRE/WTR
FRE/GRS
FRE/ICE
FRE/PSN
FRE/PSY
FRE/GHO
FLY
FLY/FTG
GHO/NRM
GHO/WTR
GHO/GRS
GHO/GRD
GHO/PSY
GHO/RCK
GHO/STL
GRS/STL
GRD/NRM
ICE/NRM
ICE/PSN
ICE/RCK
ICE/STL
NRM/PSN
NRM/RCK
NRM/STL
PSN/PSY
PSN/RCK
PSN/STL
BUG/NRM
BUG/FRE
BUG/ELC
BUG/ICE
BUG/PSY
BUG/DRG
BUG/DRK
DRK/NRM
DRK/ELC
DRK/FTG
DRK/GRD
DRK/PSY
DRK/DRG
DRK/STL
DRG/NRM
DRG/FRE
DRG/ELC
DRG/GRS
DRG/ICE
DRG/FTG
DRG/PSN
DRG/BUG
DRG/RCK
DRG/DRK
ELC/NRM
ELC/FRE
ELC/GRS
ELC/ICE
ELC/FTG
ELC/PSN
ELC/GRD
ELC/PSY
ELC/RCK
FTG/NRM
FTG/ICE
FTG/GRD
FTG/FLY
FTG/RCK
FTG/GHO
FRE/NRM
FRE/WTR
FRE/GRS
FRE/ICE
FRE/PSN
FRE/PSY
FRE/GHO
FLY
FLY/FTG
GHO/NRM
GHO/WTR
GHO/GRS
GHO/GRD
GHO/PSY
GHO/RCK
GHO/STL
GRS/STL
GRD/NRM
ICE/NRM
ICE/PSN
ICE/RCK
ICE/STL
NRM/PSN
NRM/RCK
NRM/STL
PSN/PSY
PSN/RCK
PSN/STL