If we separate the FP attractions by desirability, it is unlikely that there would be (for an appreciable length of time) any of those FP available.
TSM, Soarin', Space Mtn., Exp. Everest, Safari, Test Track, and maybe a few others, would be very hard to get.
And, frankly, I don't really care if I can schedule a FP for the "lesser" attractions.
Parades, Fireworks, Fantasmic!... those would likely figure somewhere in the "desirable" mix, too.
Character meets are a wild card, depending on guest preferences.
Consider that under the current system there are between 15k, and 20k FPs avaliable for any one particular attraction. With the exception of Magic Kingdom each of the parks have 3 or 4 marque attractions.
Marque Attractions by park : (I've probably missed something)
MK : SpaM, BTMRR, PP, Pooh, Dumbo & maybe Jungle Cruise, & SplM
EP : Sorin', TT, MS
HS : RcR, TT, TSMM, ST
AK : EE, KS, Kali, and maybe Dino and PEW.
Except on the high season days figure that 25k - 50k people are in a particular park on one particular day. If the FP+ is limited so that you can only reserve one of the marque attractions then by simple math there would be about 50% of the remaining FPs (as under the current system) avaliable. However they may be wanting to shift to a system that utilized less FPs in favor of slightly shorter standby wait times.
If they do make it where you can get a new FP+ once in the park using your smart phone, I suspect that they would have a system where limit how often you can do that. (i.e. you wouldn't be allowed to get 5 new FP+'s the moment you walk in the park). Maybe you could get one an hour after opening then another one an hour later, etc.