Random Emoji Reply
Expressing yourself through text is difficult. Most people resort to using emojis. Even then its hard to capture your emotion through a single or string of emojis. When I don't know how to response, I just send out some random emojis and let them figure out what I think. It acknowledges their message and leave them to continue the conversation.
Emoji Story Game
You can also generate a few emojis and come up the story its telling. It would be similar to James Corden's Late Late Show Emoji News segment.
Emoji are in the eye of the beholder
People generally interpret emojis differently from each other. Some are generally ambigious but also the appearance of these emojis may differ depending on the device where's its viewed. Apple, Google, Facebook, Samsung, Microsoft, etc. all have their own version of each emoji. If you think this doesn't matter, read-up on the outrage around Google's previous version of the burger and beer emojis.