There should be a skill based element to it. A numerical rank which just goes up as you earn XP plus a rating of Bronze, Silver, Gold or Platinum which is based off of how good you are relative to everyone else.
Have a leaderboard of all players for K/D, SPM and KPM.
Use each player's average leaderboard position to decide their rating.
Only count each player's last 10 hours of gameplay to ensure rating is representative of current "form".
Platinum - Top 10%
Gold - Next 20%
Silver - Next 20%
Bronze - Everyone else.
For example:
There are 100 total players in the world.
You are 10th for K/D, 17th for SPM and 12th for KPM.
This makes your average position 13th so you would be Gold Rated.
You could of course do different stats if you thought there were better, more relevant ones to pick. Personally I like those. You could also do a weighted average.
These ranks could then be used to balance servers more effectively. Even if you didn't want to show all this to the players there should definitely be something like this running in the background for that purpose. We had "skill" in BF4 but it's already been shown by myself and others years ago that whatever algorithm was calculating that was nonsense.