One thing in Fifa's favour IMO is their ethos of spreading the game - stupid in the Qatar example of course but Poland & Ukraine did host a Euro's, so perhaps an emergency relocation of 2018 to Poland, Czech Rep, Slovakia, Austria & Hungary - all bordering each other & within 90 min flight range of each other - no need for a single new stadium.
Then there's the other Eastern Europe Countries, Hungary, Croatia, Serbia, Bulgaria etc - these nations could group to host 2018. And lets face it, leaving country's with white elephant stadiums didn't stop Brazil & South Africa getting a World Cup. Of course this won't happen but it could.
For 2018 it would utterly impossible, FIFA requires 40k+ modern stadiums with loads of press seats that bring capacity down anyway plus extra space around the pitch and so on
Poland
Warsaw - National Stadium
Chorzów - Silesian Stadium
Wroclaw - Stadion Miejski
Gdańsk - PGE Arena Gdańsk
Poznań - Stadion Miejski
No problem here, five good and modern stadiums fresh from hosting Euro 2012
But then you start to struggle
Czech Republic
Not a single one close to requirements
Slovakia
Not a single one close to requirements
Austria
Only the Ernst Happel Stadium in Vienna would be capable
Hungary
Not a single one close to requirements. Would require the National Stadium to be rebuilt very quickly, albeit there are plans to do so.
So you couldn't really host it over those five.
Neither Croatia, Serbia or Bulgaria have stadiums of the required size and certainly not of the required quality to host it either.
Everywhere would have to build new stadiums, infrastructure, hotels, transport links and so on and would have nothing but massive, crippling debt to show for it afterwards, plus the clubs aren't big enough to fill what would be the new grounds either.
Admittedly looking at our stadiums a few in different cities would require new stands and new facilities built in but for the most part we're sorted. The US and Germany could host it tomorrow if they had to.