I still disagree with a permanent Thunderstorm. There are much better ways to buff it a little without changing its theme as a low-price mass CC spell:
1) Make its cost 1
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. That would make it even cheaper and able to fit into any kind of decks, regardless of element.
2) Have the player decide if he/she will strike his/her creatures with Thunderstorm or his/her opponent ones. Striking your creatures instead of your opponents may be useful sometimes (killing Antimattered creatures or Malignant cells for example).
3) Make it to delay for 1 turn every creature it hits. Now it won't be useful only against low hp creatures.
Why don't we try to think of buffs that don't ruin the low-price-mass-CC-spell theme of Thunderstorm?