I've been thinking for a long time that the profile showcase manager could be improved... and perhaps many other Steam users think so too.
These are my ideas (I hope they can be passed on to the developers):

• In showcases where you can showcase games, it's NECESSARY to add a way to simply remove a game from that showcase (such as a simple "X" to remove the game from the showcase);
  
• In showcases where you can showcase your completed games, it would be nice if, instead of having to search for the game by typing it in the search box, there could be a nice drop-down menu that only shows completed games (in the case of the collector's showcase).