'Even in C++'? You do realize how much more powerful C++ is than Actionscript, right?
The thing is, Kongregate only supports Flash games, so remaking the game in C++, besides the huge amount of time and effort needed, would also lose Elements the huge playerbase on Kong and a lot of revenue.
The game could support 2+ casting/skill cost, but it wasn't implemented for a reason: Elements should not have hybrid cards. This has been stated by Zanz and many members already. The focus of the game is elements, after all, and the elements are what should be focused on, not strange hybrid cards. Not to mention the fact that hybrid cards add another layer of complexity to what should be a simple, casually playable game.