16 teams
The first format I have in my head is a simple one, and one which everyone knows would work. Sixteen teams, split into four groups of four, and into the quarter-finals. 31 games, over less than a month, would give the public everything they could want out of a World Cup: the knowledge that the best team should have won; the chance to see less well-known international sides, and the feeling that the World Cup left them wanting more, rather than wanting to barricade themselves in a cave where they'll never again see another Super Eight match.
With sixteen teams, you would need six Associate nations. Looking at the World Cricket League Championship, they could be Afghanistan, Ireland, Kenya, Netherlands, Scotland and the UAE. Kenya would be the weakest of those sides on present form, but all of them would have at least one game that they could target as being eminently winnable, either against one of the other Associates, or a lowly Full Member. Personally, I think this is the ideal solution.
A 16-team tournament would be the ideal solution for me. This would mean a total of six Associates.
If cricket was to bring itself into line with another major sport - rugby union - then a 20-team world cup would be on the cards. This would mean that Canada. Namibia, Nepal and possibly Papua New Guinea would be likely to join the sixteen teams mentioned above, meaning that teams would play four group matches instead of five. It wouldn't change the length of the tournament much, as the 47 games could just be played with two or more per day.
However, this wouldn't really please the broadcasters very much. These extra teams would be seen as diluting the standard of the competition, providing mis-matches when they play against Full Members, and de-valuing ODI cricket when they play against fellow Associates. These aren't necessarily my views, but they would be the arguments put in the way of such a tournament for the forseeable future. A shame, really, because I'd love to see New Zealand play Papua New Guinea. And just because a match ought to be dull and one-sided, doesn't mean that it will be for sure.
The ICC would be too afraid of meagre audiences to consider expanding the tournament further.
But there would be a possible solution to the problem of diluting the talent pool. It wouldn't be a particularly popular one, because the West Indies are a part of cricket's identity, but if you invited individual West Indian countries to take part, then you could get twenty high-quality teams together. Instead of the four additional countries mentioned above, you could be looking at Barbados, Guyana, Jamaica, St Lucia and Trinidad & Tobago.
This might give you the best quality tournament, but it would obviously be thrown out because people don't want the West Indies broken up, which I completely understand. That said, though, it would likely attract audiences, especially if the tournament was in the Caribbean. So, in short, this is an option that would produce good cricket, but be about as popular as sprouts at a barbecue.
Inviting the West Indian nations separately would change the look of the tournament somewhat.
I confess, I'm playing devil's advocate now, and increasing the number of teams just for the sake of it. My 32 teams would be something along the lines of: Afghanistan, Anguilla, Antigua & Barbuda, Australia, Bangladesh, Barbados, Canada, England, Grenada, Guyana, Hong Kong, India, Ireland, Italy, Jamaica, Kenya, Namibia, Nepal, Netherlands, New Zealand, Pakistan, Papua New Guinea, Scotland,South Africa, Sri Lanka, St Kitts & Nevis, St Lucia, St Vincent & The Grenadines, Trinidad & Tobago, United Arab Emirates, United States of America, Zimbabwe. What it would certainly give you is a global tournament, exciting and showcasing cricket's standing as a global game.
Sure, a game like Australia v Italy might be a bit of a drubbing and slightly difficult to market, and okay, who's going to turn up to watch Hong Kong take on Papua New Guinea, but it's the thought that counts, right? Well, no. It's the bottom line that counts. And a 32-nation World Cup would harm the bottom line, at least in the short term. And, most worryingly of all, the group games would be pretty un-appealing. If India got grouped with Scotland, St Kitts and the United States, then they would breeze through completely un-challenged. The BCCI would be furious!
No, I think the most viable, the most saleable and the most competitive solution is the 16-team model. We've had it before, of course, in 2007, but that time it was spoiled by the infinite and infernal Super Eights. We also had Bermuda getting embarrassed a little too badly, which of course was a poor advert for cricket.
As a footnote, I think that it was actually a good advert for cricket that a tiny island off the south of Florida could come out and compete on equal terms with India, even if they get beaten. But what do I know?