fire can make air rise. (kinda obvious one but) ice freezes water. some wind attacks can break ice. Just random thoughts =D

Earth:
* Can cause fires to pop up?
* Can cause structural collapse (already in-game IIRC?)

Fire:
* Causes Air to rise and interrupts airflow

Water:
* Extinguishes Fire
* Can pool up and 'flood' an area
* Can erode Earth

Air:
* Causes Fire to heat up and expand more rapidly, 'fan the flames'

black magic can break through white magic's shield (if there is one in the first place)
and one of the white magic's attack can be used to weaken black magic users

Lightning can conduct through water? Something like that? Don’t think it would work too well.

maybe some of the fire abilities can destroy rocks from the earth magic, maybe lightning can also destroy it too

Dude just make the remainings like the wind having tornado wind blow etc. great work on the fire