You could increase the damage of eldritch blasts and make it so that applying eldritch shapes and essences reduces the damage.

So you could have the warlock gain 1d6 eldrtich blast damage every level and make it so that applying a lesser eldritch shape or essence reduces the damage by 1d6, greater shape or essence reduces damage by 2d6, and dark shape or essence by 3d6, least shapes and essences would still be free.