UPDATE: JV confirmed on Twitter this morning (5/21/15) that the 8 play set leak by Best Buy was "completely false," and that "In fact, we haven't even told retailers how many are coming for fear they would leak!
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While 8 play sets sound amazing, I think we can agree that it was also not likely.
Which begs the question (because let's face it, speculating is dangerous but fun): Just how many will there be? We know of 5 (3 Star Wars,1 Marvel, 1 Disney/Pixar), so the remaining numbers would 6 7 and 9. Despite my earlier theory, I think 9 is definitely as stretch and highly unlikely. But I could definitely see them adding 1 or maybe even 2 more play sets to the list.
Since people complained about repetitiveness of the Marvel play sets, I'm going guess that the only Marvel set we'll get is the 1 we know of with the unannounced title. And the 3 announced Star Wars play sets pretty much cover the whole saga, except for Rebels. Considering that there is a standalone Star Wars movie coming that is set during this time frame, I would guess that a play set for this era would be more likely for 4.0. There were a lot of complaints about lack of Disney play sets in 2.0, so I think any remaining toy boxes are most likely Disney (yay!).
Please remember we're all working with the words "maybe" and not "definitely" here, and that nothing we see should be believed until Disney Infinity tells us. Like I mentioned above, speculation is a dangerous (but certainly fun) hobby that can lead to disappointment. I think it's totally okay to engage in this kind of fun as long as we keep that maybe/not definitely mindset.