I think diamond leagues are not interesting anymore, as the differences between the players is too big. I usually do between 30 000 and 50 000 medals in each league, (nearly) always end up in top 5, but there is always at least 1 guy who puts out a very big score (often there are 2 or 3). I only leave diamond league when I totally stop playing (usually on holydays), and then I very easily win all my leagues until I come back to diamond league.
If you look at my latest league (see screenshot), you can clearly see that the first has nearly 10 times the amount of medals of the third, and the league record is over 10 times his score. So the leagues are mostly won by high level players, who really play a lot the game.
When you consider the latest updates, the 375 gems reward is not really interesting anymore (as an example you need to win 27 leagues to unlock the last black smith slot, and during the 3 days of the league you get half of the reward with he daily diamond chests if you bought it).
So my suggestion is to create a new league, with higher rewards (and maybe longer time too ?) of course, that would be reserved for people ending first in diamond league. That way diamond leagues would be more accessible to players and more fair.
What do you people think about it ?