I'm all for having an even 20 badges per...metal, but Collector seems like a bit of a cop-out. Furthermore, the Collector Diamond badge would mean you'd need all Bronze, Silver, Gold, and Platinum badges, since you can't have all Platinums without the Collector Platinum (which requires all Golds including Collector Gold which requires...well, you get the point). Collection prizes could be used in the form of additional content, however, like unlocking exotics/uniques to be found throughout the game, or in order to get a full victory altogether.
A Collector badge I CAN imagine is one that requires you to get all of the medals in the game (cluster, Compet-N, etc) including the hidden ones: that's worth at least a Platinum.