I noticed on the 19x19 ladder at least (not checked for the other ladders) that many players have no ongoing challenge games, and nevertheless steadily climb up the ladder, since some players ranked above them are constantly kicked off the ladder under the 3 days timeout rule, or simply quit the ladder. I think this is unfair. Of course they can be challenged any time, but they also can stay this way for ages. I suggest to add a rule that you must have at least one ongoing challenge game (either challenging or challenged), with a deadline of 3 days. If you have not engaged in a new challenge 3 days after your last game has ended, you are kicked off the ladder.
[edited one month later]. I was misled by the dynamics of the bottom 20% of the ladder where the players kicked off or leaving the ladder above you outnumber those who jump from the bottom over you. This stops further up, where you start to actually drift down again if you don’t win a challenge to jump further up. I don’t know if someone has a mathematical model of such dynamics, which I find fascinating.