WhatsApp is known to be working on Reactions. This feature is currently only available to selected beta testers. It allows you to respond to messages with an emoji as a reaction. Works similar to Facebook. The selection of emojis has so far been very limited. That could change for the final version.
This is how WABetaInfo came across behind the scenes that the selection of further emojis for the reactions is being prepared. Additional emojis for reactions can then be selected via a “+” button, as the screenshot shows.
Whether all or only selected emojis will then be available via the new button is still completely open. At the moment we can only wait.