In January 2017 the FIFA Council approved a 48 team World Cup for the 2026 edition. Up from the current 32 teams.
A lot of time is left of course and the host(s) are not chosen yet, though a joint bid by USA, Mexico and Canada is the leading candidate. However increasing the number of countries is almost certain to lower the quality of matches. It is also very difficult to come up with a good format that is not as straightforward as 32 (and other numbers that are power of two)
FIFA has already agreed on the format and the plan is to have 16 groups of three teams with two team advancing. This already makes the group stage meaningless and arguably unfair. A three group means three games per group, take for example,
Team A vs Team B
Team B vs Team C
Team A vs Team C
How will a group like this be scheduled without team having more rest days than the other? Or that the last match can have both teams qualifying with a mutually beneficial result.
The allocation is another issue and as mentioned can cause a weakening and dilution of the tournament.
Asia will have 8 slots instead of the current 4.5
Africa 9 instead of 5
North, central America 6 instead 3.5
South America 6 instead of 4.5
Oceania 1 instead of 0.5
Europe 16 instead of 13
Plus 2 more slots in as yet undetermined playoffs, which can depend on the number of hosts as well.
An existing football infrastructure or hosts capable of building it is another barrier that is at least not lessened by this kind of expansion. It remains to be seen if this effects the number of serious bids that are already few enough as it is. FIFA already requires at least one 80,000 capacity stadium and more with 60,000. The final decision on the hosts is expected in 2020.