Chrome OS notifications are getting a new card-style look it seems. The new notification style will still pop up from the bottom right corner of the screen as before, but the cards will now have a slightly flatter design and be all white with black and grey text, and dark grey icons to denote the app that the notification is for. This isn't a huge departure from the way that Chrome OS notifications currently look, but the cards are a lot shorter and longer now as opposed to the taller and fatter design, and the app icons on the current cards are larger and displayed in color just as they would be on Android. These new cards provide a more simplistic visual style for the notifications that should be less distracting as the focus isn't on the colorful designs of the icons, but merely about the context of the notifications.
The new notifications design seems to be available as flag you have to enable though it's likely only available in the canary channel as it's noted that this is where the flag was found. If you're looking at the different chrome flags, the one for the new notifications is listed as "new style notification," and if you enable the flag you'll need a quick reboot of Chrome or the Chromebook entirely, then the notifications that do come in from that point on should have the style of the notifications shown off in the screenshot images below.
Since this seems to be a canary channel feature only at the moment there's no telling how long it will be before it gets moved up the chain through the dev channel and beta channel builds before making it to the stable channel, and that's also assuming it even makes it all the way to the stable channel in the first place, as the canary channel is experimental and not everything tested in this build gets used. That said, this is one of a few changes that have been popping up in the canary channel as of late. Recently, canary channel builds for Chromebooks started receiving a new material design style lock screen.