I like the look of this and really hope it gets implemented. However, I do see where @DrBloodmoney is coming from. Both this and the current leaderboard would be pretty easily exploited in short time, either by playing tons of ratalaika shovelware, or focusing on easy quick ultra rare games( boosting ancient ps3 games, garbage that was given free on plus etc.) Heck one could even climb both very rapidly if they could tolerate playing these type of games a lot.
However the average trophy hunter would be left in the dust on both leaderboards if just playing typical games regardless of the amount of effort given or the rate of which they are completing them. It would be nice if there were some way to measure this effort as well somehow.
This rarity leaderboard would be a great addition to the site, but it isn't the solution to the ezpz problem that many are looking for.