EDIT: Please refer to my more recent posts on the Map Completion rewards Adjustments as there are better suggestions and the idea of guaranteed black lion key is not as great as an idea as I initially thought - https://en-forum.guildwars2.com/discussion/comment/446784/#Comment_446784
Currently when you complete a map you have a chance to get a transmutation charge or a Black Lion Key (excluding City maps and Southsun Cove which only give a single transmutation charge).
I think most people can agree that getting a transmutation charge rather than a black lion key, this is doubly irritating when you remember that the latest iteration of the black lion chest gives you a chance of 5 transmutation charges, its incredibly disappointing.
I would suggest removing Transmutation charges from the map completion reward and make it a guaranteed Black Lion key or provide an amount of the Black Lion Statuettes, at least in the case of statuettes you can buy 2 transmutation charges or you can save them up for greater rewards.
I feel like this kind of change would incentivise people to do map completion more, and maybe encourage people to buy Black Lion keys because they're closer to that item they wanted rather than through frustration.
If you have any suggestions of your own, feel free to post them below