No need to change the format at this stage but if you want to increase to 24 teams just add one team to each pool and then add one week to the competition.
For qualification so we get a fair draw, this would be my way of doings things. For qualifying, I would combine the Asian and Pacific qualification process as opposed to previous qualifiers. And then I would guarantee one team from each region. Then based on how the other teams in each region go in qualifying, I would allocate 4 Repechage spots. For this there could be 4 Repechage qualifying pools of say 4 teams. So lets take an example.
Lets say from this world cup the Pools end up finishing like the following (just random guesses):
Pool A - Australia, Wales, England, Fiji, Uruguay
Pool B - South Africa, Scotland, Japan, Samoa, USA
Pool C - New Zealand, Argentina, Georgia, Tonga, Namibia
Pool D - Ireland, France, Italy, Romania, Canada (unless Romania and France win today)
So through go the Australia, Wales, England, South Africa, Scotland, Japan, New Zealand, Argentina, Georgia, Ireland, France, Italy.
Then the regional take place. Lets say the regional qualifiers are:
Europe: Romania, Oceania/Asia: Samoa, Americas: USA, Africa: Namibia
Then you take the next best four teams from each region. So for example (based on what we saw last time):
Europe: Russia, Portugal, Spain, Belgium
Oceania/Asia: Fiji, Tonga, Hong Kong, Korea (I assume the other Pacific Nations wouldn't mount a big challenge).
Americas: Canada, Uruguay, Chile, Brazil
Africa: Zimbabwe, Kenya, Madagascar, Tunisia.
Then you allocate them a Repechage Pool based on their World Ranking. Do this in a random draw. So the tiers for these pools would be similar to that of the actual World Cup Draw.
So according to the rankings today, the tiers would be like this:
Tier 1: Fiji, Tonga, Canada, Uruguay
Tier 2: Spain, Russia, Chile, Hong Kong
Tier 3: Korea, Belgium, Zimbabwe, Kenya
Tier 4: Portugal, Brazil, Tunisia, Madagascar,
So for example the Pools could be:
Pool A - Fiji, Hong Kong, Belgium, Brazil
Pool B - Canada, Russia, Korea, Madagascar
Pool C - Tonga, Spain, Kenya, Tunisia
Pool D - Uruguay, Chile, Zimbabwe, Portugal
These games can be played at a random venue - possibly in Japan this time around as they are hosting the world cup. Then the winner of these Pools each go through to the World Cup. Then when you want to expand the tournament to 24 teams, the top two from these Pools will go through.
Finally, when everyone has qualified, the draw will for the World Cup will be done. This can be done a year before the tournament and when everyone has qualified. The rankings of every single team can be taken into account, so teams like Fiji (who are ranked 10th in the world but probably won't gain automatic qualification) can be put into the 3rd tier for the draw.
I hope this isn't overly complicated haha.
