How long is the shortest Apple Keynote that's been done in recent years? They are normally going for 2 hours+. The one exception in recent history is the iPad Pro 9,7-inch keynote. That one was short, it took just over one hour.
If Apple is only going to show a MacBook 13-inch (identical to the 12-inch, only slightly larger?), new MacBook Pro 13-inch and new MacBook Pro 15-inch they will have to really stretch things if they are going to hit the same length that they normally do with these keynotes.
macOS 10.12 Sierra, iOS 10, watchOS 3 and tvOS 10 was released only a short while ago and Apple have already dedicated previous keynotes to talk about these releases so I doubt there will be much talk about software during this keynote. I won't be surprised to see them releasing 10.12.2 BETA, iOS 10.2 BETA, watchOS 3.2 BETA and tvOS 10.1/10.0.2 BETA but I highly doubt they will include anything really special so they feel the need for showing it during the keynote.
I have the feeling that there has to be something more under the wraps here? A whole keynote just for showing of three new MacBook's sounds awkward. It might be that they had planned to show of a new Cinema Display and iMac's, but rumours has it that they got delayed until Q1 2017. So perhaps the keynote gets handicapped as a result of this, who knows?
Or perhaps they will refresh other things like the Apple AirPort Express? It's getting pretty old and not having 802.11ac support on the Express makes it really dated as a access point. 802.11ac 2x2 chipsets are really cheap, so Apple should be able to update it with 802.11ac and still keep the same price point. And perhaps they could included updated AirPlay specs with better support for playing in multiple rooms directly from iOS-devices? Or perhaps their partnership with Sonos might make it so that they could include Sonos within the AirPort Express making it into a really cheap and awesome alternative to the Sonos Connect.
They could also update the AirPort Time Capsule, we have gotten newer revisions and generations of the 802.11ac spec so a new Time Capsule could support better 802.11ac speeds, and faster SoC's for improved routing performance and better performance towards the built-in hard drive for faster Time Machine backups.
One thing that is bugging me is Apple's attitude towards hardware like the Mac mini. I do understand that the Mac mini is not something they focus on anymore. Desktop computing like the Mac mini is not really something that sells all that well anymore, it makes much more sense to focus on the iMac etc.. But they don't need to redo the Mac mini, but for the love of god put up-to-date hardware within it. Just refresh the hardware into Intel Skylake, DDR4 and add support for Apple's NVMe SSD's instead of regular old SATA SSD's and your golden. The price / performance of the Mac mini is plain stupid when it features two (about to become three) generation old hardware and still have the same price tag as it did when it got released in 2014.
I would love to grab a Mac mini to replace my Time Capsule as a Time Machine backup server. Backing up to a Time Capsule is painfully slow, running macOS Server on a Mac mini and stick 2x 1TB or 2TB hard drives in RAID1 inside of it would work out much better. I have no problem spending the amount of money Apple wants for the Mac mini in order to achieve this, but I won't pay that price when the hardware is that old.