The unstable gas is anything but final...
have you seen how much damage an 18 Air Pillar, 6 Nymphs, 6 Unstable Gasses and Mark of Fire can do?
I quite like the idea of the Entropy bomb. It is a bit, what's the word... child-like though. Pass the bomb, explode it or pass it back, sort of thing... It could be nice, and yeah the Time-Bomb definitely needs a lower counter number.
I can see unstable gas being nerfed in a way in which it works like these ideas here.
When you play Unstable Gas, it enters the game with 3 counters and every turn it gets +3 counters.
For Counters/3 Fire you can ignite it causes Counters damage to your opponent and Counters/9 damage to each creature in play.