Day 1's Steam universe announcement was SteamOS. Valve completely blew our original predictions out of the water by basically announcing/hinting to all of them. Except for the actual Steambox.
Steambox
Someone came up with an interesting comparison. I think I first read it on Reddit but it was something along the lines of this:
Valve is going to pull the same thing Google is doing with Android. They (in collab with another manufacturer) make and launch a "Nexus" device to set a standard for newer devices, and third party manufacturers make devices to compete with that one.
This is a really interesting theory. It matches up with what Gabe Newell has said in the past, and what the SteamOS page revealed yesterday;
SteamOS will be available soon as a free download for users and as a freely licensable operating system for manufacturers.
I think this is the most likely scenario for Wednesday's announcement. Valve is going to show a (or a bunch of) Steamboxes, made in collaboration with third-party manufacturers.
Is it a coincidence AMD has a press conference planned a mere 2 hours after Valve announces whatever Day 2 is? Probably.
Edited 25/9: Maybe not.
How about VP of Epic Games Mark Rein recently seeing the "most amazing thing" by NVIDIA? Maybe.
Maybe NVIDIA and/or AMD will be making the Steambox equivalent of a Google Nexus device. Valve won't be able to completely do it on their own, so it makes sense for them to get help from one of these established names.
In possibly related news, NVIDIA has started helping open-source NVIDIA driver alternative project "Nouveau". Interesting development.
Edited 25/9:Linus Torvalds, father of the Linux kernel has responded to Ars Technica regarding NVIDIA helping out Nouveau;
Torvalds has responded to Ars, saying he's optimistic but not quite ready to apologize to Nvidia. "We'll see," Torvalds wrote in an e-mail. "I'm cautiously optimistic that this is a real shift in how Nvidia perceives Linux. The actual docs released so far are fairly limited, and in themselves they wouldn't be a big thing, but if Nvidia really does follow up and start opening up more, that would certainly be great."They've already been much better in the ARM SoC space than they were on the more traditional GPU side, and I really hope that some day I can just apologize for ever giving them the finger."
A Controller
Valve has not been silent about trying to developer their own controller. The announcement page has a controller on the floor (and a cat staring at it). If they announce a Steambox, they will most likely announce a controller as well.
A (somewhat long) while back, a patent from Valve describing a modular controller popped up. Very interesting concept, but it's pretty old and might not be relevant anymore.
There's also this app called "Steam Controller Configs". It's probably for syncing your controller configurations to the cloud, which could of course be with any controller.
Then there's the Steam Hardware group. Maybe it'll be a group for people with a Steambox to discuss things? It did pop up around the same time the "hometest" repository did. Or maybe it's a remnant of Valve's hardware team of which a large portion "left" Valve earlier this year.
More recently, Valve updated one of the packages in the hometest repository and added some USB device rules. Two stood out, since the USB vendor ID they use (28de) is still unknown. This ID might belong to Valve and could be further proof of Valve working on their own controller.
#USB devices
SUBSYSTEM=="usb", ATTRS{idVendor}=="c251", ATTRS{idProduct}=="2202", MODE="0666"
SUBSYSTEM=="usb", ATTRS{idVendor}=="28de", ATTRS{idProduct}=="2202", MODE="0666"
SUBSYSTEM=="usb", ATTRS{idVendor}=="28de", ATTRS{idProduct}=="1101", MODE="0666"
Something to launch Steambox with
A while back, it leaked that Left 4 Dead 3 was in development, and would be the first game (that we know of) utilizing Source 2. Naturally, there's another game Valve fans are waiting for but other than a project group on Valve's issue tracker we really don't know if this is in development or not.
However, Marc Laidlaw (writer at Valve) clearly stated that the announcements would be Steam related. This makes it look somewhat unlikely that Valve is going to announce a game this week.
Maybe in the next few weeks the following quote from the SteamOS page will become a reality. But with a Valve game.
Watch for announcements in the coming weeks about all the AAA titles coming natively to SteamOS in 2014.
– Marlamin who has started seeing things due to Steam induced insomnia