ancient anchor
This functionality would be like this: the boxes we have remain like this but with the name "common" and if you merge 2 of these common boxes (of the same type) a rare box comes out; with 2 rare ones, an epic one comes out; And with 2 epics you would get a legendary box.
The improvement in rewards could be in one of these ways:
1)As they are merged, lower rarity rewards disappear (example: in a rare box, common rewards disappear and in an epic box, rare rewards disappear)
2)Like previous rarity boxes, as it improves, the chance of rare things increases and the chance of common things decreases, up to a point where the common and rare rarities disappear.
Temporary boxes (like the current Valentine's Day box) should also be able to be merged, so that players don't have to open as many boxes to get special rewards.