The only mind I currently have to add a new tab on the leaderboard is

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  Reputations

  
  


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  Limited Items.

  
  

To add a little more explanation to this, the limited items tab will count how many limited items that a user has in total (besides if the user's inventory is private), but for the reputations, well you know it's a feature on the forums, no need to explain this lol.

the rep idea would be good to implement but other users could - rep others to gain places for no reason or go on alts to disadvantage others

Yeah, they could either add a 50-day or 4-week limit, meaning that every month, users can give reputation to the same person again. To prevent rep-farming exactly.

(or not every month since I mentioned the 4-week limit when it's the 1st day of a another month)

Rep leaderboard would be cool. Idk about limiteds tho

i dont think it'd be a good idea to make having higher rep a competition, i like how the forums are fairly peaceful