A 64 team world cup, eh? Blimey.
I can't see you'd get many nations, or even coherent groups of nations, able to host that with the max 2 stadia/city and 40k+ capacity requirements. Even with the support of the GAA in Ireland and if Norn Iron gets it's shit together over their national stadium, I think you'd struggle to find ~24 suitable grounds in the British (& Irish) Isles.
I'd guess that means the group stage being spread all over the world. Which, when there's pretty obviously a fucktonne of problems coming from climate change, seems a bit ... frivolous.