IMO, if the World Cup is going to expand (and I tend to agree with FreshFish that it will not), they would push it to 36, and do 6 groups of 6 (top 2 advance from each group plus the top 4 third place teams). The problem with groups of 5 is that not all of the teams can play on the same day for the final game day. As a result, I think groups of 4 or 6 are the only real solutions. 6 groups of 6 still allows all the top 2 teams to advance (9 groups of 4 would mean that 2 second place finishers would miss), while still keeping the traditional 2nd Round - QF - SF - Finals format. I think having the top 8 teams have byes into the 2nd Round is basically a non-starter. I think a pure knockout tournament (a la the 1934 World Cup) would be more likely.
The obvious issue with having groups of 6 though is that it basically adds another week or so to the tournament (I don't think that would be seen in a good light for all the world clubs). Currently, they play 48 group stage games over 2 weeks, with 3 games/day through the first two games, and then 4 games/day for the final game of each group. A move to groups of 6 would require 90 group stage games. Even if you did 4 games/day through the first 4 group games, and then 6 games/day for the final game of each group, it would take 3 weeks. Plus, you have the likely issue of the extra strain on the host to accommodate the extra fans/games. It's doable, but unlikely.
Ultimately, the only thing that would likely happen as a result of expansion would be that more UEFA teams would get in. If 36 were the option, my guess is at least 2 (if not 2.5 or 3) of the extra spots would go to UEFA, with Oceania getting a full spot. I think FIFA is happy with how many teams are currently going from each confederation, and I don't see them changing that up all too much.