I have suggestions for league promotion :
Case 1: you need to be the very best
Req:
minimum form top 5 players in the league or top 20% from the league (if the league only consists of 5 players, you need to be in a 1st rank. But if the league consists of 100 players, you need to be the tip 5 players)
Ranking 1 need to win 3 battles in a row from top 20% of the players in the league.
Ranking 2 win 4 battles, ranking 3 win 5 battles, ranking 4 win 6 battles, ranking 5 win 7 battles. If you're on the 5th rank, win the battle and get much points to be ranked up, for example became 1st rank, you only need to win 2 more battles. In a new rank, it counts only 1 battle from the lower rank. So there's no difference if you got ranked up from 5th to 1st from 1 or 2 battles, it only count as 1 point to get promotion. But if you're from the 6th rank to the 5th rank, you still need to win 7 battles.
Battle from outside of top 20% will not be counted, win nor lose. But if you lose, you can be kicked from the top 5 or top 20%.
In this case, the minimum league players is 10, and you need to beat the 2nd rank 3 times to get promotion.
Case 2: frequent user
Req:
You have fight at least maximum from 500 battles or 5x the number of the league players.
Victory rates are higher than the average of the victory rates in the league.
Case 3: league promotion give away.
You make an event for specific time period and specific rules.
By that, the players don't need to wait a specific time to get promoted and have a goal to achieve.
And you got more loyal players to build up the game.