Just an idea, not sure if its been thought before:

Have a tier 3 skill to unlock "chest trade":
once unlocked you can trade a certain number (say 25) silver chests for one event chest of your choice. Or keep it strictly Jeweled chest: trade 2 jeweled for one silver event; something of that sort.

This will help further players upgrade their event characters (the mission system is good, but the 1 hero you can't choose as the gear option makes it difficult to build your ideal build)

Vingummi, 25 silvers for a SILVER event chest are fine. For a JEWELED one, well, maybe not 1000, but definitely more than 2-300.