Most likely the uber duper super gimped and cut down version of Windows 10 will be that nice free upgrade everybody is hyping so much. The other versions (the ones not terribly gimped or cut down or dumbed down) will be yours for some absurd ammount of money (your good ol' Microsoft as usual). That is probably your catch.
Or probably play the F2P game and deconstruct their entire marketing philosophy, making the core OS free but everything else (and I mean lots of software we take for granted and are usually shipped as part of the very OS) worth some bucks being cashed in a microtransaction. Leave the OS as barebones as they can (W10 shipped with IE1X, just to be used for downloading Spartan or Chrome, for example) and attempt to pull a moneygrab with almost everything else. Also try to separate software suites ala Android and make rip Office apart, making Word cost 5 bucks, Excel another 5, Powerpoint another 10, and that other crap I never install worth a little less, but make all of them individual installs.
My bet is option 1, option 2 would be nice (I just want the OS, I use office only for compatibility reasons, I wouldnt touch the rest of their software withy a 10 ft pole) but then again, this discussion is only relevant for those who actually pay for their Windows installation. The other 2/3rds of the population will most probably have another dilemmas regarding installing this new version of Windows (gotta give a really good reason to make those W7/XP fans change their minds, you know).