Give each player highest stat a different colour to their name, so when their magic is the highest level, their name turns purple in the playerlist (for example)

Ill likely do this by properly implementing advancement criterions, so you can build your own advancements that run functions and grant the colors

This will be doable in the next release, for example creating an advancement with a criterion of

"criteria": {
    "levelup": {
      "trigger": "rpgstats:player_level",
      "conditions": {
        "stat": "rpgstats:defence",
        "level": 5
      }
    }
  }

will fire when the player reaches defence lv 5 (also "rpgstats:_any" for any stat)