The version of this idea I play with is this:
Ruined Village
$3 - Action
+1 Card
+1 Action
--
You may play this during your Action phase even if you have no actions left.
I like it, but this, too, mucks with the game rules a bit, so that is a disadvantage. (Although I don't believe there are any conflicts yet.)
I think it's really clever and innovative to make this a Treasure card rather than an Action card, because not only does it offer this same sort of anti-dead-drawing mechanic, but it provides a way to play Action cards after Treasure cards, which is something that has great combo potential but is currently only possible with the idiosyncratic Black Market card.
I think there's a real downside to having it cost $6, though. The core purpose of the card -- that is, what makes it better than a Market, which is itself only an adequate $5 card -- is to finagle a Village effect out of it. But it's usually not practical to try to build an engine out of a $6 Village (Border Village is an exception because what you're really buying is a $5 engine component with a bonus Village to go along with it).
So I would recommend dropping the coin and the buy so as to be able to reduce the price:
Lucky Coin
$3 - Treasure
Worth $0
Immediately begin a new action phase.
+1 Card
+1 Action
Then the name "Lucky Coin" might not be as appropriate, since it no longer gives you a coin to spend, but I dunno -- who says lucky coins have to have monetary value?
iangoth's suggestion of making it a Reaction card is the cleanest way to make it work within the Dominion rules, but you'd lose the Black Market effect by making it an Action card instead of a Treasure card, which would be unfortunate.