just make bigger chance for everyone to win trophies, that will keep players in game, easily, as i said many times, it can be maded in two ways, in my opinion
Simply all trophie winners put in same leagues, there they can decide who is bigger fish, non trophie teams in leagues where they can get more fairly chance to raise trophies
Second way is to change algoritams and make leagues like associations or king event, look on % average of 15 best players in team than make leagues of same power teams 10% diff up and down no matter of manager lvl