Heres just an idea. What if we could upgrade walls with Dark Elixir? Level 9+ only. Now, before you say this is too easy, I mean like 25k Dark Elixir per wall, or maybe even more. This would only cater to TH9's, and above, because when you are at the level of farming level 10 walls, chances are, you are maxing heroes as well. So, you can either choose between heroes, or walls. This may not be a good idea, but let me know what you guys think.

