It might be a good idea to make Shard of Serendipity the shard of the "Other" element and create a new Shard for Entropy.
Proposed Changes: Shard of Serendipity 3
Unupgraded: Add 3 random unupgraded cards to your hand
Upgraded: Add 3 upgraded cards to your hand
New Shard for Entropy:
Preferably something that would be useful in an mono-entropy deck and not just in Rainbows.
Suggestions:
Antimatter a random creature each turn. If destroyed, all creatures antimattered by this shard are antimattered again. (i.e. they will regain their stats)
Each turn, shard destroys a random creature or permanent an opponent controls
Each turn, your creatures receive a random buff: 25% chance +0/+0, 25% chance +1/+0, 25% chance +0/+1, 25% chance +1/+1 (stackable, creatures still attack)
Each turn, generates a random Entropy creature, casts a random Entropy spell or creates a random Entropy permanent