Any extra DPS ability should be weighted against resources, MP and cast time in a way where finding the time and resources to cast it rewards the healer's careful planning.

I'd say give each healer a very expensive, long cast time spell, either dot, or direct damage, that prevents them form weaving anything in it and requires to stay stationary for a while-

(In case of AST for example, rather than a damaging spell it could be a short lived single target buff)