I have a few ideas that my guildies and I have come up with for a solution to the stock piling of seemingly useless Black Lion Chests.
If you want people to actually use them, and therefore purchase more gems/keys, you have to make the rewards worth it. Rather than increasing the drop rate of keys, (which we all still wish you would), how about putting things in them that would make them worth opening. Nobody wants Mystery Tonics. High level crafting materials would be more useful and desired, such as 10 Oricalchum, Gossamer, Jewelry Orbs, etc, or 3-5 Obsidian Shards, Ectos, Lodestones, etc, or 1 Clover.
Even the ability to take shattered and broken pieces of Lockpicks to the Mystic Forge to upgrade them to Black lion Keys, would be great instead of just increasing the drop rate of keys.
Mix up the rewards a little. Maybe even throw in some recipes for the Mystic Forge or an exotic every once in a while. But the Mystery Tonics and never-ending transmution stones have deemed these chests… useless.