Considering current install bases of the new gen machines, and that on 360 and PS3 it will not sell huge numbers I thing 10-12 million across all consoles, and potentially 15M+ if it comes to PC.
If Destiny gets the critical praise it's aiming for then I can see Destiny 2 selling mega millions but it won't be on PS360, so it still won't be able to get much beyond 20 million even with a PC release.
MW3 is pretty much maxing out the audience for FPS and with digital PC sales that probably got to something above 35 million. I think by the time Destiny 3 comes out there might be a big enough PS4bone install base to approach 30 million, along with PC.
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