In your very interesting article "Complexity in Rules on Cards" (http://forum.dominionstrategy.com/index.php?topic=121.0), you have stated the Vanilla card problem.
For example Dominion does not have a card that just says "+1 Card +1 Action +$1." If I made that card, it would limit what other cards I could make. So instead I just do variations on it.
In the Fan Cards Forum, people discuss sometimes about this. What exactly is the reason not to make a one-dollar-cantrip for $4? Would it be too strong at $4 but too weak at $5? Or would it be okay at $4 but just boring?
A $4 Peddler (are we clear on what I mean here, yes, okay) would probably be fine. I mean it would be good, but it would be acceptable power-level-wise.
The Vanilla card problem is, vanilla cards limit what other cards can do (in games where cards can't naturally just be blatantly different in power level). For example Village at $3 means Village-with-a-bonus has to cost $4+. Otherwise some people are unhappy. It is really about that unhappiness, not necessarily any other issues. If say Walled Village cost $3, there would be people who were pissed. It would have no other negative consequences; it's still worth avoiding.
So, again just in certain kinds of games, you want to ration out the vanilla cards. They're valuable for being very simple cards, but limit what other cards can do.
My feeling was that $4-Peddler was a good one not to do. I did it, right away, but didn't use it; the main set got Market instead. With Market I can replace the +1 Buy with a different bonus and get a parade of cards that aren't clearly better/worse across the board. And then at $4 there are a couple cards that are some mix of better and worse than $4-Peddler, although that makes them more complex cards.
In the end, well the decision not to do $4-Peddler was an early one, and the years did not produce as many comparable $4's as expected. It probably would have been fine to have had $4-Peddler exist all these years. Tournament is obviously both better and worse; Ironmonger has acceptable differences. And Peddler itself you are hoping to pay $0 for.