I was thinking maybe there could be spells in the Spell Factory that could be created with Dark Elixir rather than regular elixir. There could be new spells that are created by Dark Elixir and have some sort of "Special Effect" on troops fueled by Dark Elixir. I don't have any suggestions for any new spells but it's just an idea. A new building like a Dark Spell Factory could even be created. Regular Spells could also be created spending Dark Elixir.
