Well, that's what I was actually wondering, myself. You see, If my memory serves, 2013 was when SKT won the World Championship in NA, and as you stated, 2014 was Korea. 2015 we went to EU (Germany and London and Paris(?)) and then in 2016 they brought it back to NA. 2017 was in China, so if we are looking at major regions, I think EU or Korea are a shoe in for Worlds 2018.
HOWEVER, NOW! There is the Wildcard regions to consider for 2018, too. Riot has spent a lot more time looking into those regions (Brazil, CIS, OCE, etc.) and we could be looking at a Worlds being hosted in a place like Rio de Janeiro. I doubt we will ever see a Worlds event in OCE, as it is a very small region and much of the crowd would have to come from outside of that area (flown in) I feel. But maybe, since OCE is still a considerable region. I think it'll be interesting to see where they take us next, but I wouldn't count Korea out by any means.