It seems like dark mode is on its way to WhatsApp for Android, at least according to the latest info provided by WABetaInfo. The source is well-known for app teardowns, and while checking the v2.18.281 of the app, the latest one that arrived via the Google Play Beta Program, the source managed to find out that WhatsApp seems to be preparing for dark mode introduction, though that's not all, the 'Swipe to Reply' feature also seems to be on the way.
The dark mode option will probably be quite similar as on a number of other apps, though it remains to be seen if WhatsApp will opt for a pure black background that is OLED-compatible, or will it opt for a dark gray, or something of the sort. In any case, we can only guess at this point, but the update is on its way, that's for sure. Now, as far as 'Swipe to Reply' is concerned, the source did share a GIF image, showing how that feature works, though if you've ever used WhatsApp on iOS, you're probably quite familiar with it, as it has been available on iOS for years at this point. With this feature, WhatsApp will automatically load the message in the reply contest, as soon as you swipe the message.
The last change that the source has been able to dig out has to do with group chats. If there are more than 10 participants in a group chat, a 'More' button will appear, and by clicking on it, you'll be able to see a full list of people that are in a specific group. As already mentioned, we're still not sure when will these changes arrive to Android, though it's possible that they won't all arrive at the same time. The source is quite confident we'll get them soon, so stay tuned for that, and keep in mind that they are not available in the beta version of the app just yet either, all of the info that we've mentioned here was taken from the code of the beta app, basically.