You guys have some interesting ideas. This might belong in the Creative forum, but I'll give my thoughts.
1) Thunderstorm: I like the idea of an Arena wide attack spell. I think that would make for a good epic attack.
2) Lava: I do not like the idea of a persistent Attack Spell like this. I think your idea would better be represented by a Conjuration. You conjure up a Lava Pool which immediately makes an attack against all non-flying objects in the zone and make the attack again during every Upkeep Phase or whenever a creature enters the zone.
3) Tsunami: The water school needs more attack spells, period. A Zone attack and a level 2 ranged attack are minimum for it to really compete with the other schools. I'm not sure from your description why Tsunami would require being epic though. Is it another Arena wide attack?
4) Tornado: Again I don't like the persistent effect, and I don't like how it arbitrarily changes the rules for Bash attacks.
5) Fissure: Another spell that is better represented by a Conjuration.
6) Volcanic Eruption: A lot of complicated rules to fit on one card and something else I believe would work better as a Conjuration.
7) Acid Rain: I like this concept.
When thinking about possible attack spells, remember that they are not objects and do not remain in play. It is part of their definition that they are immediately discarded after causing an effect. If you want something that persists on the game board, it is probably better represented by another spell type.
Overall, I like the conversation. Can't wait to see what other ideas are came up with.