As I've said before here, one thing that will be essential for both shows, is that Demons learn that they are NOT naturally evil. It's the system that's been established that encourages and tempts them to go bad. Plus, it keeps demons fighting each other. Because if they were unified, they could threaten the harmful status quo Heaven endorses.
One method of solving this systemic issue in Hell is learning who is behind it. So far, I have 2 suspects.
Roo. As the ultimate villain of HH and maybe HB, it's Roo is has corrupted Hell. Plus, she's a greater scope villain yet to show up. And greater scope villains tend to be THAT powerful.
2 Lilith is said to have empowered Hell with her songs. But if our fears of her being a villainess are true, it's possible that her songs contains seeds to discord and disharmony. And those effects still power Hell.
Additionally, it could be possible that BOTH are responsible. Either working together or Lilith is Roo's pawn. Regardless, it should be established that this doesn't MAKE demons be bad. But it does plant ideas in their heads. Perhaps a harmonious song from Charlie could solve this? Bonus points if Lilith is a pawn, and both she and Charlie sing a harmonious song which solves this.
Regardless, Demons will still be capable of doing bad. But without Roo or Lilith's influence, fewer ideas for being bad will be planted in their heads.
Anyway, this is MY theory. Got any theories of your own?