Hmm... You could also set aside your discard pile on play (i think moving an entire pile should be okay, Chancellor does it). The hard part is getting it back. You could probably do an effect on leaving play (under a line), but i don't know whether that's really an improvement.
I think this leads to more rules confusion than the current version. Where do discarded cards go when you set aside the Discard pile? They can't go to your set aside Discard pile, so you would have to create a new Discard pile, right? Man, that's confusing.
Well, i didn't mean that the set-aside discard pile was still "the discard pile". Chancellor moving your deck in your discard pile doesn't mean you draw from your discard pile now (you kind of do after shuffeling, but you get what i mean - it's not like you take the top 3 cards from your discard when you play Smithy). Anyhow, the fact that you might set aside cards after playing Furnace simply eluded me.
Then again, you could simply set aside the discard pile first:
Furnace
Type: Action
Cost: 5$
Set aside your discard pile.
+2 cards.
If you drew 2 cards this way, play an Action card from your hand as if it were a Furnace (or reveal a hand with no Action cards). It's a Furnace until it leaves play. Put the cards you set aside with this in your discard pile at this turn's clean-up phase.
<My usual rambling about how to improve cards by using discarding>
Here's a shorter version:
Furnace
Type: Action
Cost: 5$
Set aside your discard pile.
+2 cards.
If you drew 2 cards this way, discard an action card (or reveal a hand with no action cards). If you did, play this again. Put the cards you set aside with this in your discard pile at this turn's clean-up phase.
And one that's even shorter, but doesn't stop eating your cards when the pile is empty:
Furnace
Type: Action
Cost: 5$
Discard an action card (or reveal a hand with no action cards) and set aside your discard pile. If you did both:
+2 cards and play this again. Put the cards you set aside this way in your discard pile at this turn's clean-up phase.
Edit: Wow, i posted three versions of your card... It's kind of like i'm trying to steal this thread for me, and a bit like i did with LF's Smithy-Village... Sorry. I'll tone this behaviour down in the future.