Problem I see is that it would require double targeting, which I've heard is trouble.
I don't think it's double targeting. It's a single targeting that triggers another targeting.
From a code point of view is different, because Zanz call the targeting of Imp Spirit that has a single target. That triggers the effect of the card targeted, that can has target or not (Schrodinger's Cat, for example, does not have a target).
It's not very simple, but technically is not double targeting, and i think it can be done.
Other than that, i think that we should not bother with technical limitations, because if a card too complicated it's Zanz choice.
Back to the card: I really like it!!! But... let's take an example: Gargoyle and Lycanthrope.
If Imp Spirit use Gargoyle's ability or Lycanthrope's ability, they lose their ability as they used it, or they retain it?
I think they should lose it, like if they used it.
And they should get the bonus, because that bonuses has no target, so you cannot change it.
About Unstable Gas: It should be destroyed like a normal activation.
To better clarify that, i would use the wording "Trigger target ability. You choose eventual target."
