I think clash of clans is missing a lot of clash Royale characters. If only all the characters were in both games; there would be a lot more excitement, more things that could go unpredictably and more.
That would get very confusing very fast. A lot of characters works for Royale because there are only 8 cards in a deck. In Clash of Clans, you have 280 capacity's worth of troops. That's just far too many options for you to use in an attack, and it would get very confusing, especially for newer players.