A recently spotted series of commits to the Chromium Gerrit seems to suggest that Chromebooks will feature easy-to-use Windows dual boot options at some point in the future. Specifically, the changes are filed under the internal codename "Campfire" as well as directly with Google's own Pixelbook offerings via a firmware branch referred to as "eve-campfire." The latter of those references cropped up in another commit back in April alongside a reference to an "AltOS" and HLK Testing. The first of those seems to point to an alternate operating system and was originally thought to be tied in with Fuchsia OS. On the other hand, HLK is now thought to indicate that the device being tested was aiming to achieve Windows hardware certification. Taken together, the implication is that the Pixelbook in question will be able to boot up both Windows and Chrome OS.
Further commits have appeared in the meantime, adding substantial weight to the speculation. What is perhaps the most direct correlation can be drawn from a debug commit that partitions internal space for "WINDATA." That's set to a minimum of 30GB, while 10GB remains for the primary system, Chrome OS. Getting started with dual-booting on the Pixelbook also doesn't appear to require the Developer Channel or Developer Mode. Instead, it is directly supported at the OS level via signed updates to the "RW_LEGACY" portion of the stock ROM. It's also turned on through the chrome shell via a relatively straightforward command, which the average user should be able to easily enable.
Simultaneously, the option to dual-boot into Windows doesn't appear to be something that will only be supported on Google's Pixelbook. While each device that can support the feature will still likely require the corresponding hardware certification and other prerequisites, yet another commit indicates that there will be at least a few "campfire variants." That doesn't mean that every Chromebook will be able to boot into Windows since at least one requirement is that the device in question features more than 50GB of storage. However, it does open up the possibility that more users will be able to take advantage of the feature on future hardware. For the time being, Google hasn't made any statements to confirm or deny that its Pixelbook or other Chromebooks will be able to dual-boot with Windows. However, the addition of Windows on top of Linux and Android app support would be a serious bolster to the already strong OS.