I'm guessing they will go right to the meat of things and upgrade one of their MacBook Pro models. That is right up the "Performance per watt" avenue which Apple claims their new silicon is best for. They are going to want to squash any rumors that these don't perform well right off the bat so I don't think they are going to be conservative here.
For similar reasons, I don't think the iMac was ever slated to be the first Apple Silicon machine. The desktop form-factor just doesn't highlight the benefits of the new architecture the way a laptop does. Since they sell a lot more laptops than desktops, it's likely they don't even have a desktop specific CPU ready at launch time. That'll come next year or maybe even 2022 towards the tail end of their 2 year launch window.