WHM - the DoT proccing something would be interesting.

SCH - for sure needs bane and Miasma back. A hard cast spell would be nice.

AST - if the card system were robust enough it could get away with only having 1 DoT like in SB. Otherwise the damage compile ability Aravell mentioned would be fun.

SGE - maybe something like a reverse Kardion? Something that deals damage when you cast a heal? This one's a tough one to really think of something fun. Hopefully the Eukrasian Dyskrasia stacks for funsies.