Have 3 Chest at the end of the boss fight. Add 1 key drop each successful encounter you run through (getting max score)
To make it fair so people don't botch other peoples runs, have the keys only usable for that run. Please upvote so mods can see this. 1 chest after one completion is too little for the effort especially for heroic. This is the reason the reckoning sucks (granted you can get the exact weapon and masterwork you want from menagerie) I hate doing more than 3 runs of the reckoning just for a chance to get a weapon and even then it won't happen. No one is going to want to run menagerie over and over for 1+ gear
Edit: Make one of those chest or the first chest a powerful drop while you're next trip gives you the second powerful drop
Please UPVOTE so mods can see.
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I like this idea, but the fix won't be the end of the world. Imagine if the glitch hadn't existed to begin with. Menagerie would still be an amazing way to farm what you want. Faster than forges, easier than Reckoning, and is easily more fun than either.