I will break this down for those of you unaware of how the matchmaking system functions…
Halo 5 uses a variant of the ELO rating system. The variation comes from an algorithm, blending of two different numbers - your MMR (matchmaking ranking) and your actual Halo rank. Your MMR is a hidden number on the backend. Let’s say you are Diamond in Feburary season in team arena. When you start your initial 10 game ranking the next season, the system knows your MMR from last season, and matches you accordingly. Now lets say you have a few bad games in the qualifying rounds and you place into gold. 10 games isnt enough to destabilize your MMR, so the system thinks you belong in diamond - you will find yourself ranking up through gold much faster than you would in Diamond (going up for 5th and 6th place in FFA, that kinda thing). It’s sort of a magnet effect until you reach the rank that lines up with your MMR.
This works fine for team arena, etc (team based modes) because ranking up and down is primarily based on win or lose. This doesn’t work as well in FFA.
Here’s an example. Take a look at the CSR population breakdown in FFA - http://halotracker.com/h5/csrbreakdown#Free-for-All
64% of the population is currently unranked in FFA for this season.
The system right now thinks my MMR is Onyx 1850. Unfortunately, I am more likely to encounter unranked players in my games than ranked. Because of this, if I place 2nd in a FFA game against mostly unranked players (with MMR at my level), I gain little to no CSR. However, if I place average or poorly in a game in the same situation, I am heavily penalized and take a significant CSR hit. If you are provided little to no reward for playing well against players of your skill level, and punished heavily for playing poorly against players of your skill level, there is no way this model is sustainable.
IMO here’s the best solution: Leave ranking system untouched for Diamond and down. For Onyx/Champ, base the win/loss reward/penalty strictly on what your opponents MMR is.