If there are enough people why not split into two equally sized games, and then randomise the players again each round. That way you're likely to encounter different players each round, and so it will be harder to bear a grudge?
Could even be 3 or 4 concurrent games depending on the number of players?
I think 5 groups of 5 players with 5 rounds would be good, it's possible to rotate players so that nobody plays the same person twice in 1 "season"
Also 5 player games will likely be more tactical, be quicker, and also less likely to have a lot of players eliminated early and have to wait for next round...
Just some thoughts :)
5 player diplo is much easier to make and balance too :)
Well if you want to run all 24 players on one map I've just finished the Scenario I was making for this league, it's here if you want to take a look anyway :)