WhatsApp's newest incentive may come as surprise to you as it is to me as well, but something possibly quite useful is on its way.
But before giving you any further details, first, it is time for a little bit of a backstory. So, to this moment, stickers from third-party keyboard apps that have their custom implementations are being sent as static images instead. To resolve this minor problem, WhatsApp has decided to partner with keyboard app developers to allow deeper integration, and thus provide a totally seamless experience, whether you want to use the built-in stickers or any other alternatives.
As recently discovered by @WABetaInfo, the first keyboard app to add support for this feature is Google's own Gboard. According to him, the feature has been enabled in WhatsApp since December 12 to give Google time to work on their implementation, though as of now there are no possibilities to try it out until it goes live in the following weeks.
At this time there is no info about other keyboards testing this, probably because WhatsApp has provided instructions only to Google, but based on @WABetaInfo's statement, we may see other keyboard apps with this implementation in the near future.