It seems to me that locked chests do not reset or respawn. The locked doors reset every 10 minutes so I think it would be cool if every 10 minutes the game checks to see the number and difficulty levels of chests that have been unlocked. Then an equal number of chests with the appropriate difficulty level would lock or relock in the same area.
This example would utilize the static chests that already exist in game
A rouge in dungeon 'A' unlocks 2 skill level 10 chests and 3 skill level 20 chests
And
A rouge in dungeon 'B' unlocks 1 skill level 20 chest, 2 skill level 30 chests, and a skill level 50 chest
10 minutes later. . .
The game checks to see which chests have been unlocked
Dungeon 'A' resets 2 skill level 10 and 3 skill level 20 chests
while dungeon 'B' resets 1 skill leve 20, 2 skill level 30, and 1 skill level 50 chests
One last suggestion is that either:
1) The same chests could always relock to the same difficulty (a little more boring)
2) Unlocked chests would switch to a different chest in the same area (adds a little more thrill to the hunt)