Friendly Trash Talk
Perhaps my favorite feature in Clash Royale is the pre-set communications between two players while fighting. During a battle there's a little chat box in the corner, and there are four "emotions" that make sound, and six different things you can say to the other player. Like, "Good Luck", "Well Played", and others, or even a crying face for sore losers.
Of course Clash of Clans we aren't fighting in real-time, and the enemy typically watches the replay later to see how much damage was done by an attack. However, it would still be neat to have similar options. Leave a smiley face after getting a 3-star attack, say "Well Played" or "Good Defense" if an attack fails, or any communication for that matter.
This would be especially nice in Clan Wars, where two different Clans fight against each-other. Being able to say small little things to the enemy Clan, or even enabling some form of "Clan War Chat" would be greatly appreciated by the entire community.
Right now Clash Royale only has six things to say to the other player, and I'd love a few more to show my frustration or excitement when winning, but people would also abuse this power. There are limits, but adding a few more to either game certainly would be a plus.