Though I think the treasure bags for mini bosses and event bosses aren't necessary, the masks would make a nice addition. One of the things users eventually try to do after they've beaten every boss, done every event, tried every class and class approach, and in all modes nonetheless is collect. Masks and banners are one of those items that seem like they're there for the sole purpose of being collected, and not having a complete collection for boss masks is something I've wondered about myself. They're vanity items, and just a simplified sprite for a boss's sprite, so it shouldn't be that hard to implement and work on as a small side project. It's a perfectly reasonable ask for the developers to complete the set.