With a 20 team World cup, you could use the Rugby World Cup format and have 4 groups of 5 teams.
Top 2 team qualify for the Quarter-Finals.
A 4-game group stage means everyone gets a good run in this competition but only the best 8 get to contest the final prize
Edit:
While I'm at it. I was always a huge fan of the old Heineken Cup quarter final seeding system.
Each of the qualified teams are ranked into a table using their group stats (Points, Goal difference, goals for, ect)
and the teams are drawn as such.
1st vs 8th
2nd vs 7th
3rd vs 6th
4th vs 5th
It means that even while the groups are wide open, people can have fun thinking about who they might get if results go their way. Or what kind of a result they need to avoid certain players. For the fans, this system was ****** amazing and very exciting.
While I like the randomness of unseeded draws used in football. There's a certain magic to the old Heineken cup seeding system