Simple one. Move the requirement from 8 lore to 6 lore.
Because of the requirements, RDD builds tend to be pretty limited. RDD typically wants at least 10 levels, and it requires 3 levels of bard/sorcerer at minimum. However, because of BAB requirements for Blackguard/Liberator, this presents a very awkward cutoff for them, unable to pickup divine synergy while also snagging 10 levels of RDD.
It would be a small and simple change, and make them a lot more viable than they currently are.