I'd imagine Karma's innate ability would be a benefit to Apocalypse, since theoretically she could act as any of the Horsemen. She'd need distinct enhancements to augment it and actually act as whatever role she played, but as Famine for example she could possess workers at a food factory and force them to destroy the goods, or poison it.
Husk I think is a good candidate for either Famine or Pestilence, given her mutant ability (she can "shed" layers of skin and alter the composition of the new layer, thereby becoming stone or metal and so forth. She can also shed wounded layers and heal quickly, unless the wound is deep. Husk's counterpart in the Age of Apocalypse could create chemicals, liquids, gases, and even weapons like blades, and could choose to alter only part of her body).