Hmm. Nice mechanic. Except that it plays in the Miracle space and might have to change the way people draw their cards forevermore, but Wizards did already cross that bridge.

Well, you'd really only have to change the way you draw if your deck included Sift cards. I think Sift is rather strong, of course, especially when I put it on cards like Sand Elemental and Anydrian Wastes, so it's possible lots of decks would include it.

Well, the 'can't double sift' does at least provide balance. One Anydrian Wastes is better than Blasted Landscape. But if I sift a Wastes into a Wastes, I'm stuck. Can't say the same about the landscape.

what does it mean you can't sift that card?

It means that if that card has Sift, you can't do it. I thought that was pretty self-explanatory.

It was obvious to me, but amuseum's right that it isn't gramatically correct. Not unless this Sift is both a keyword, and a keyword action at the same time. Perhaps the card should be written as "When you draw this card, you may sift it" with reminder text, so you can then establish you can't sift again?

Then again, a number of cards say "When you cycle this card", so maybe I'm overthinking it.

Try do it like Miracle, it will make it work.

You may reveal this card from your hand as your draw it. When you reveal this card this way you may put it on the bottom of your library and draw a card. You can't Sift a card drawn with Sift.

Maybe?

Patrik got it here. The issue is what is "that card"? It's not obvious that you may draw another sift card. But it seems that is exactly what you're trying to prevent. So why not just say "If you do, put the top card of your library into your hand" to avoid multiple draw replacement effects and the sifting chains you feared.

I worded it this way because I thought it was more intuitive, but apparently that's not the case.
For now I'll change it from "As you draw this card, you may reveal it and put it on the bottom of your library. If you do, draw a card. You can't sift that card" to "You may reveal this card and put it on the bottom of your library when you draw it. If you do, put the top card of your library into your hand."