Does anyone actually like any of these systems or are we just trying to find the most acceptable system from a format that gives room for no good options? For me, there's no doubt it's the latter of these options.
But if I have to accept that there will most likely be 24 teams in 2016, I haven't really made up my mind about what version I prefer, but let me list the options given so far and give them a reality check.
20 teams
4 groups of 5 teams The two best teams of each group will go to the quarter finals. 7 games to win the tournament. Major issue: rest days (also on the final match day).
5 groups of 4 teams Winners plus the best runner up goes to QF's; remaining four runner ups play off for the final two QF spots. 6/7 games to win the tournament. Major issues: separation of teams in the same position across groups, teams will play a different number of games to reach a certain stage, rest days.
5 groups of 4 teams Winners to QF's, runner ups and 3rd's play off for QF spots. 6/7 games to win the tournament. Major issues: teams will play a different number of games to reach a certain stage, rest days.
24 teams
4 groups of 6 teams 2 best to QF's or best 4 to round of 16 (not specified). 8/9 games to win the tournament depending on system. Major issues: match congestion, number of games per team.
6 groups of 4 teams 2 best and 4 best 3rd placed teams to round of 16. 7 games to win the tournament. Similar to WC86-94. Major issue: separation of teams in the same position across groups.
6 groups of 4 teams 2 best to reach a new group stage with 4 groups of 3 teams. Second round group winners to semi final. 7 games to win the tournament. Similar to WC82. Major issues: two group phases, rest days (also on the final match day).
6 groups of 4 teams 4 best group winners to QF's, remaining 2 group winners and all second placed teams will play off for 4 QF spots. 6/7 games to win the tournament. Major issues: separation of teams in the same position across groups, teams will play a different number of games to reach a certain stage, rest days.
I think I've covered all suggested systems now. I don't think 20 teams is an option, but I've included them for the sake of completion. Let me now look at the issues and give my opinion on them.
Major issue: rest days (also on the final match day). With this issue, one team will have a bye day in every round and one team will therefore have finished its schedule before the last match day. Teams will be unequally rested for games. This allows some teams to play on results and some teams can just sit helpless on the final match day crossing their fingers for right results in other games.
Major issue: separation of teams in the same position across groups. Here we compare incomparable teams. It is rather common to do this, but it's still a horrible idea; you can be punished for being in an evenly balanced group.
Major issue: teams will play a different number of games to reach a certain stage. In such a short competition, it might have a major impact on results that teams will have to play a different number of games in order to reach a certain stage, and this is not an obvious advantage for the teams who have done well early on as numerous examples from teams who have rested players in the final group match show (among this 3 out of 4 in the recent EURO).
Major issue: match congestion. This gives a lot of games every day and some matches will have to be played at the same time. This takes away some of the charm of being able to watch (almost) all games live during such a tournament.
Major issue: number of games per team. Playing more than 7 games would either mean that the tournament is prolonged or that teams will have to play a lot of games in a very short time span. This comes on top of a long club season.
Major issue: two group phases. It takes a long time before we get to the knock out matches.
For me, all of these issues are negative as compared to what we have now. Having looked at all of the propsed systems, I'm split between two. Either the WC86-94 system or the system with play offs between 2 group winners and the second placed teams. However, I don't like that you single out group winners as having performed worse than other group winners, I think I prefer this to happen to 3rd placed teams as none of them can hardly claim to have performed brilliantly, no matter what. This leads me to prefer the WC86-94 system (did I just write that???) even with all the flaws it has. One that I didn't mention is that the group phase eliminates very few teams, so you can scrape through with very mediocre results (Italy, anyone?)
The fact that I prefer the WC86-94 system is great evidence that expanding the EURO to 24 teams is an idea so stupid that it actually makes Sepp Blatter look relatively sane with some of his proposals
Actually, my preferred system is the one with 20 teams in 5 groups and 2nd and 3rd placed teams playing off for QF spots, but since I don't see a tournament with 20 teams, I didn't consider it much.