Some of the flavor for Primal heroes is that they have the patronage of some spirit whose power they channel. This is most obvious with shaman where the spirit manifests as a creature on the battlefield, and the druid, where they take the spirit's form, but barbarians channel this power when raging and wardens have limited shape-changing (via their daily powers). Druids in 4e get an extra at-will power so that they can always have options both in caster form and beast form; one possible option for a power source distinction would be to give all Primal heroes some sort of manifestation of their spirit and then separate powers that are usable by the spirit and the hero.

Also, going to the original etymology of warlock as "oath-breaker" (that is breaking the covenant with God), they could be reflavored a bit to be that they stole power from a spirit (either through trickery or betrayal) rather than the current flavor of making a pact with an extra-planar entity (ironic given the origin of the term). This might given them access to effects that are usually Primal (because they are channelling the power from a spirit), but otherwise have the chassis of an Arcane class (since they imprisoned it with Arcane magic).