There are some things so far missing from discussion about what Apple’s going to announce today, which seem worth noting here, not because they’ll necessarily happen, but because it might be worth reflecting on them if they don’t.
HomeKit
Remember HomeKit? Announced at WWDC 2014, it was going to be the way for apps to control smart items around your dumb house (or perhaps the other way round). Things have been awfully quiet around this, despite many manufacturers being keyed up for it as far back as January at CES. (Thanks @lessien for the reminder on this.)
Apple TV
Long overdue an update, it will anyway have to get some sort of software update so that it can be controlled by the Apple Watch. Remember how Tim Cook said that he controlled his Apple TV with his watch? You won’t be able to do that without a software update, so one is surely coming there. Quite probably this will wrap together with the HomeKit update.
Photos
There have been betas of the new Photos app (to replace the ageing iPhoto) kicking up and down the web for over a month now, so it must be time to have something concrete. This seems like a good time to do it – perhaps it would also beam them to, oh, a watch?
“Retina” MacBook Air
There was all the fuss around this a couple of months back, and now it’s gone quiet. This feels like it would be a distraction from the main event, and doesn’t fit into the flow above – HomeKit links to Apple TV links to Apple Watch, and Photos is expected – but it’s one to perhaps consider.
Apple Pay
Apple has a team in Europe who have been working with banks on implementing Apple Pay; if they’ve been doing the right work then they could have it on track to get started as soon as April in the UK, just in time for the Watch to go on sale and be used by all those people who have iPhone 5S, 6 or 6 Plus models. It would be quite a coup, and no doubt would see all sorts of records being broken. Again, this would fit into the flow, but I’m not sure how likely it is. There haven’t been any murmurings, but then again, banks are quite good at keeping secrets (such as how they’ve reduced the interest rate on your savings again while offering great rates to people joining the bank.)
I don’t expect anything around iTunes Radio or Beats. Although something on that is getting overdue, it just doesn’t gel with the whole Watch theme.
The Overspill: when there's more that I want to say 
I only expect Apple to talk about the Apple Watch and many new apps that will support the device. No Apple TV. No MBA. Nothing else. Apple likes to send focused messages.
Fair enough. Though as I say, these are the loose ends that will also need consideration – particularly the Apple TV update and HomeKit, where the latter has languished.