Given the well-understood issues with making an Action-Treasure [...]
Sorry, I've missed or forgotten that discussion; what's wrong with it just being played as both? It's an Action-Treasure, so when you played it, you played an Action, and you played a Treasure.
Hmm. I've seen extensive discussion of it before. I thought it was in a guide for fan cards, but I can't find it any more. )-8
The first question would be: would it have one effect played as an Action and another played as a Treasure, or just one effect you could play as either? (Playing "as an Action" meaning something different from playing "an Action" - you'd be playing an Action, even when you played it as a Treasure, I guess.)
One big objection is that it would no longer be the case that if it was yellow and on the table, it was money to spend that turn, because you might have played it as an Action not a Treasure. Though I guess Counterfeit, Coin of the Realm and Storyteller have already complicated that more than somewhat in their respective ways.
There would have to be clarification of whether "Action card you have in play" really meant that or actually intended "Card you have in play as an Action" for Peddler, "Treasure card you have in play" similarly for Bank (and, I suppose the corresponding question relating to Conspirator after Storyteller). If the "Actions in play" and "Treasures in play" were left meaning literally that, the new Action -Treasure would have to be balanced in the face of them.
I note that we're however many expansions in, and an Action-Treasure has never happened before, despite it being an obvious combination even back in the days of Intrigue's Action-Victory and Treasure-Victory cards. It seems pretty clear to me that Donald X has seen some objections to making them before, and has found a way to overcome those objections now.