Water / Flying
Grass / Steel
Fire / Fairy
Water/Flying : this typing is only represented my Pelipper and Gyarados (and Mantine and Swanna and Cramorant but they aren't really strong so they aren't taken in account lol) and I think we can do a lot, and very original things with it.
Grass/Steel: only seen on Kartana and Ferrothorn, which have very specific movepools and stats making them strong. it seems very interesting in CAP meta too
Fire/Fairy: an extra combination of cool types sounds nice, with lots of possibilities too