While Google's upcoming event on October 4th may have the most excitement lined up in regards to new products, Google is seemingly planning to share new details about their Cardboard VR viewers before this event, as Google's UK Twitter account has recently teased that they would be sharing something new that has to do with Cardboard on September 29th. The tweet, which is rather nondescript, doesn't say much beyond the dates, which are from September 29th to October 1st. Having said that, a date that spans a few days may suggest that Google has multiple things to share about Cardboard and that they will be doing it over the span of those dates.
While it's entirely possible that Google may be looking to reveal a new version of the Cardboard viewer in the next few days, there is no actual evidence of this from the tweet itself. Being this close to the reveal of a bunch of other new products though, does seem to point in that direction. Then again Google could simply have something else to share about Cardboard, such as new partnerships and content that could be enjoyed on the vast selection of viewers that are based on the Cardboard build and technology, as well as on the official Cardboard viewer from Google.
Whether or not Google UK has an event planned for these listed dates is unclear, and if they do have an event planned Google is keeping things quiet. In regards to new Cardboard compatible content, popular Augmented Reality game Pokémon GO was rumored to potentially be gaining Cardboard support in the near future, and while it's probably a reach, being able to play the game through your Google Cardboard viewer does seem like something that could be worthy of a mysterious tweet followed by an announcement. That said, there is really just no way of knowing what Google UK's plans will be in the next couple of days, Perhaps those dates will be nothing more than a series of additional teases relating to Cardboard, with an actual unveiling of what those teases are referring to happening on October 4th with the rest of the stuff Google is due to announce.