Buddy distance collector
Niantic should give 1 free one to each player, you can store up to 10 kilometers, but you can only have 1 for 1 buddy. When you switch your buddy, your lost distance will be collected. And when you switch to the same buddy again, you can add collected distance to the buddy distance. It has no effect on the total distance walked. A pack of 3 can be bought with 225 PokéCoins in the in-game shop.
Answers
For any new feature you need to ask if it can be abused.
For this, we get one collector for each legendary you want candy for. You walk it for 19.5kms - then store it.
The next time we have an event that is 'half buddy distance' and 'double candy' you get them out.
You get 3 candies instantly and in 500 meters you get another.
That is 8 Legendary candy for every km walked, until you run out of 'collectors'
Just a thought...
