I think that can be a good idea to introduce a new resource: Diamond. It can be similar to the Dark Elixir, with only one storage available from TH lvl 7, and with 1 mine with TH lvl 8, a second one with TH 9 and a third one with TH 10, same characteristics of the Dark Elixir.
Diamond can be used for new troops in a special barrack called "Light Barrack" or "Diamond Barrack", it's the same, and also used to create not an hero like barbarian king and archer queen but an immortal troop that can be similar to them but not the same called the "old wizard": you have to build the podium that can cost 10'000 diamond and then you can use it only if you pay 500 diamond. No upgrades and it can be used only to attack not to defense, it also occupates like an healer, different from the barbarian king that doesn't occupate space in the army camp.
It isn't an easy idea but I hope you enjoy it.

