Depends on what the powers that be want from the World Cups.
A. Do they want continuity in the heartlands of the big Tier one nations which guarantee big crowds and lots of money in established rugby cities and regions.
B. Do they want to expand the game to a wider audience in Tier 2 nations or smaller Tier 1 nations. After all there are tier 2 nations with the facilities and infrastructure to host at short notice such as Germany, Spain, Portugal, USA etc. The only question would be would the local communities buy into it or
would you have a half full 60k seater stadium to watch a game?
For me the RWC should be cycled into a large Tier 1 nations hosts followed by a Tier 2 nation and so on. If RWC 2023 is in France or Ireland then 2027 has to be outside Europe in North or South America. If 2023 is in SA then perhaps Germany or Spain or Portugal.