Google introduced Allo and Duo at the same time, but while Allo is getting shut down, Duo is getting some new features.
Google Duo is gaining a new group calling feature and low light mode. Screenshots shared by Android Police give us an early glimpse at the features, revealing that group calling will support calls with up to seven people at once. You'll first need to create a group with the people you want on the call, then you can start a video call with them at any time. Unfortunately, you can't add anyone to a group during a call, but that may change in the future.
Meanwhile, low light mode is pretty much what it sounds like. When you're in a dark room, you'll see a pop-up asking if you'd like to enable low light mode. The feature "automatically adjusts your video in dark environments so others can see you better."
Both of these features appear to be in early testing right now, so they're not yet available in a setting toggle that you can just flip or anything like that. This leak does suggest that Google is getting closer to launching them to the public, though, so if you use Google Duo, you've got these new features to look forward to.