Hello. I have a few ideas that You may consider
1. A new building - an Exchange - as in currency exchange you can for exakple exchange some elixir into a gold, of course not in 1:1, let's say that for a 1M you will get 800K, you must always pay 1/5 of what you exchanging (for gold and elixir). For dark elixir ratio should be 1:200 - 1M gold/elixir gives you 5K dark elixir; if you exchange 5K dark elixir, you will get 1M gold/elixir, and so on.
Ruled out.
2. A new spell - Invisibility. Yes, I know that many times it was suggested, but here's a catch - once you will use Invisibility spell, your units within its radius will loose 50% damage power! Of course it should come out from the dark spell factory.
Maybe, I'm not sure though. Creates too many complications IMO.
3. A new unit - I was thinking about some kind of equivalent for a wallbreaker from dark barracks, maybe some kind of a mole? Depending on the level of this unit, it can dig 3-7 squares. It should have 5 levels: lvl 1 - 3 squares, lvl 2 - 4 squares and so on. Once you will reach max level for this unit, it will aleays dig under 7 squares, not less!
Not really sure what you are trying to get at here.
What do you think about that?
Cheers!
P.