In light of rinkworks's very nice piece of guide, let me add something which I think is also very important for the variant to be interesting, at least in my opinion:
1. do something different. Now that we have 100+ canal cards already, I don't think yet another smithy variant would raise a lot of interest. Nor will some combination of parts of some cards. For example, while it is intrinsically interesting to think about how a +4 cards discard 2 card will fare, in the end it will probably work mostly like smithy or envoy. Personally I am not that interested in a detailed comparison of the performance of similar cards, especially when they are not even official cards.
2. don't give out curse easily. In fact, I think Young Witch is the only cursing card that is properly balanced. All the previous cursing cards are overpowered (maybe except Torturer without +actions) and limit the strategic space of a game when they are present. I think my recent experience in veto mode confirms this: games are generally more interesting without those cursing cards, after the initial shiny period for new players.
3. Try to have interesting interactions with existing cards. If we think about the most recent expansion, menagerie is a very good example. It not only interacts with other cards in the same set, but also interacts very nicely with discarding attacks.
Here's an example I did a while ago that I still find interesting:
-----------------------------------------
Credit Card
Action-Duration
Cost 0?
When you buy this card, you may pay extra coins for it. For every coin you pay for it, put one credit token on the "Personal Credit History"(PCH) mat.
This turn:
+1 action,
choose any x between 0 and 5.
+x coins, and put x credit tokens on the card.
Next turn:
Put another credit token on the card. You may move any number of the credit tokens on the card to the PCH mat, if you remove three times the credit tokens from the PCH mat. If there is no more credit token on the card, discard it from play.
During the action phase of this turn, you may discard any number of treasures any time just before you play an action. Whenever you discard a treasure of value x, pick a Credit Card in play, and move x tokens on it to the PCH mat. If there are no more credit tokens on the card, discard the card from play.
If you did not play any Credit Card this turn, and there is any Credit Card in play, skip your buy phase this turn.
If this card is still in play in the clean up phase, do not move it to the discard pile. At the start of next turn, do everything listed under "Next turn".
At the end of the game, every six credit tokens on the PCH mat worth 1 VP.
----------------------------------------------------