One problem with this approach to "Bane" cards is that they're heavily dependent on the strength of the Bane. If the Bane is, say, Governor, then this makes Governor even more overpowered (you can upgrade your Estates into Farmer's Markets, buy them when you only have $4, etc). Similarly, Fields is awesome if the Bane is Duke, Distant Lands, or Rebuild. But if the Bane is Counting House, then the cards you've made are very weak.
Young Witch, after all, doesn't directly interact with the Bane. Sure, if the Bane is Lighthouse, then YW is pretty worthless, and it's also weakened if the bane is a spammable card like Menagerie or Scheme, but normally, YW's strength depends on the rest of the Kingdom more than the strength of the Bane.
Edit: Ninja'd by Awaclus.